One of my players had a question that I couldn't answer find an answer for. He was wondering, if he made the requisite costs (minus envervation, in this case), could he simply "add" the special ability of the Sword of Life-Drinking (http://www.d20pfsrd.com/magic-items/magic-weapons/specific-magic-weapons/s word-of-life-stealing) to his katana?
My initial reaction was, "hell no!" Haha, but I couldn't find any rule that said he couldn't in a quick perusal of the core rulebook.
Would this be GM caveat or is there a rule that says otherwise?
Well, I'd say it falls to GM caveat, since RAW you can't really 'extract' the abilities of a specific weapon and apply them to your own custom weapon.
RAI, it can be done. If you isolate the cost of the desired property to its base value, you use that as a cost to apply to the weapon like a set gold amount. Of course, you could simply treat it as a Base Price Bonus Property and round it to the most appropriate number (+1, +2, etc.)
In addition, any pre-requisites for item creation that the crafter does not meet (aside from the item creation feat) only adds a +5 to the Craft DC needed to succeed.
With your example, the weapon is a +2 Longsword with the listed property. A +2 Weapon is 8,000 Gold Pieces (without other costs).
The total item is worth 25,715. Subtracting base cost and Masterwork Quality (315 Gold Pieces), you're left with the 25,400 - 8,000 = 17,400 Gold Pieces as a cost, which is akin from going to a +2 Weapon to a +3 Weapon.
You could simply make it a 17,400 Gold Cost property, or you could make it a +1 Base Price Property.
17,400 is close to the total for a +3, not the difference between a +3 and a +2.
The difference between a +2 and a +3 is 10,000.
The difference between a +2 and a +4 is 24,000.
This is somewhere in between. You could price it as +17,400 gp, a +1 property plus an extra 7,400 gp (a bit odd), or a +1.5 or +1.6 property (going from +2 to +3.5 is 16,500 gp, to +3.6 is 17,920), or you could round it up to a +2 if you want to apply some premium.
Of course, if you just want a katana of life stealing instead of a longsword of life stealing, it's much easier. Just add the extra 35 gp (the difference between a longsword and a katana).
But yeah, there's no RAW on this. It's all GM territory.