By the rules, I don't think so.
But that's not to say you can't convince your DM to let you do it, similar to the upgrading feature of WoW D20:
Use the Craft Table to find the cost of a +1 composite longbow (200gp). Then use it to find the cost of a +2 composite longbow (300gp). Then, subtract the price of the old from the new, leaving you with 100gp. Convert it to silver pieces, which is 1000sp. From there, find the DC of a +2 Composite Longbow (19).
Then, make craft checks as normal at DC 19, only you need to go to 333sp instead of 1000sp, since you're just 'upgrading' it instead of starting over with a new bow. If I allowed this rule as a DM, which I probably would, I'd rule that the +1 composite longbow to be upgraded must be crafted by the player first. No store-bought ones.
That help you out at all...?