That's what I've been doing. If you have a bow that's built for increased Strength, you can add up to that maximum number to AP provided you have the Str for it.
So, if you have a Bossonian longbow (AP 5*) rated for a Str 18 character (+4) and your Str just happens to be 18, then the bow has an AP rating of 9*. If your Str is 16, the bow has an AP of 8*, and you get the standard attack penalties for using a bow that's too stout for you.