Yes, but you should not. In most cases, anyways, unless the enemy has really, really terrible AC and you have a very high BAB.
Because if you use natural attacks with iteratives, they become secondary attacks with a large penalty (-5). So while you may get more attacks, all of them are way less likely to hit.
I crunched the numbers with my lvl 10 bloodrager lizardfolk. With low AC, my average damage using improved unarmed attack was about the same as when just using my natural attacks. And against more normal AC, I was losing a lot of DPR.
Yes, you can combine natural weapons with unarmed strikes. As long as the limbs used to perform the unarmed strikes are not the same as the ones used to perform the natural attacks you're good to go. Unarmed strikes are treated as manufactured weapons in this regard.
However, as per the rules of mixing manufactured weapons with natural attacks, all of your natural weapons are treated as secondary natural attacks meaning they will suffer a -5 to hit and only get 1/2 your strength modifier to damage. Your DM may allow you to pick up the feat multi-attack which would reduce the penalty to hit from a -5 to a -2.
Regardless, TWF penalties would not apply to your natural weapons.
If his natural attacks were already secondary, that probably changes everything. My guy attacks with a +10 str modifier, on 3 primary natural attacks, so turning those into secondary attacks greatly lowered the output per attack and made it really bad vs. DR enemies.
BAB penalties or no, having more attacks does up your chances to hit if for no other reason than you're rolling more dice. But that's assuming you can hit the opponent by rolling a 10. If you have to roll a 16 or higher to hit at full BAB, then you might not want to lower your attack bonuses on the majority of your attacks.
In my case I was turning 3 natural attacks at full BAB and full str (+10) and power attack for 1 attack at full BAB and 4 at -5 and all at half strength and half power attack. The amulet of mighty fist bonus benefits from more attacks but overall each attack did almost half as much on a hit with a 25% lower chance of hitting.
Taking all of the feats like multiattack only made it average out to "about the same". So why take a bunch of feats to do about the same in the best of cases and do worse in the rest? In my case the goal was to fish for more natural attacks, not manufactured weapons or unarmed strikes.
But the key element here is that lizardfolk get 3 primary natural weapons. If all you have are secondary natural weapons, then you may as well stack 'em with manufactured or unarmed attacks. But in those cases you should be going for monk or brawler, I don't recall which is best.
So why take a bunch of feats to do about the same in the best of cases and do worse in the rest?I was thinking in terms of a Druidzilla character getting Attacks through Wildshape and increasing the Damage/Attack by taking levels in Warpriest. Warpriests inflict Sacred Weapon Damage instead of regular NA Damage for weapons they have Weapon Focus in. If you have a Human, Half Elven, or Half Orc Warpriest, you can take Martial Versatility and apply feats like Weapon Focus to all the Attacks in any single weapon Group. Unarmed Strikes are in both the Close and Natural fighter weapon group. So that might not be a reason to actually combine Unarmed Strikes in a Natural Full Attack, but there is a reason for some Natural Attacking characters to invest Feats into Unarmed Strikes. Another thing to consider is that sometimes a Druid like that will be in Human form, and then it would be nice if she had some decent melee capability.
In my case the goal was to fish for more natural attacks, not manufactured weapons or unarmed strikes.How many did you get, and how did you get them?
Every build will require its own calculations.
Right now with my 3 natural attacks I think I have the highest DPR of the party, and I also grapple and coup de grace like a total beast. But my defenses are bad so that's the next priority. But the most obvious way to increase my DPR will be with casting Monstrous Extremities twice every morning to gain 2 hoof attacks. There are probably ways to get wing attacks or tail attacks as well but I am not sure.
Yes you can use Two-Weapon Fighting with unarmed strikes and also use natural weapons. However, there are a few drawbacks:
1) As mentioned, at best you are going to at -2 on all attacks (-2 on TWF unarmed strikes and -2 with secondary natural attacks using Multiattack).
2) Unless a brawler, monk, or similar class/archetype, or taking the Double Slice feat, the "off-hand" unarmed strikes will only be able to add half Str bonus to damage. I don't recall a way to get full Str bonus on secondary natural attacks.
3) Unarmed strikes and natural weapons generally have poor critical ranges/multipliers, which become very important factors for DPR starting around 7th-8th character level.
4) Attacking some foes with unarmed strikes and/or natural weapons is... not a good idea (oozes, certain undead, etc.).
On the other hand (pun intended), there is one benefit to this type of character: They only need to invest in an amulet of mighty fists for all their melee attacks.