Change a weapon to adamantium?

s there any way in RAW to permanently change a weapon to adamantium?

A weapon has to be created as a masterwork weapon from adamantine. You can't just convert an existing weapon to a special material.

Except for Wolverine :-)

Is it already a magic weapon?

If not, sell it and buy an adamantine version, then enchant it.

If it is magic, talk to your GM and see if he will allow you to simply pay the 3000 gp it costs to make a weapon out of adamantine and he can flavor it as some magic ritual that costs 3000 gp worth of adamantine that infuses into your weapon.

With the rules as they are though, no. There is no way to make a non-adamantine weapon into adamantine permanently. And its for the best because otherwise you could buy many cheap weapons, convert them to adamantine and sell them for much more than you bought them.

While there is no way to do it RAW, you could create something along the lines of masterwork transformation that instead transforms an item's base material.

It should be higher level (I guess 4th or 5th for adamantine, one less for mithral) and should cost at least as much in material components as the difference in price would be.

Possibly RAW would allow, with GM's discretion, use of a Limited Wish:

"Produce any other effect whose power level is in line with the above effects" is a pretty broad catch-all for the caster to "wish" for anything he wants and let the GM sort it out. Me, I would have no problems with this, although I would expect the caster to provide the adamantium, just like he would have to provide expensive spell components (per the description of Limited Wish).

Given that, I'd say a 7th level spell should be able to do this, but you're GM may see things differently.

You'd have to cast Lord Darkwind's Uncanny Bonding. :)