Modifiy the source?

Hello,

I did read thru the portage manual and my memory is failing me,too.
I will install gentoo in the next couple of days again (HD died on me - who needs backups anyway?).
The thing is that I would like to put a couple of different drivers in xfree and the kernel that are not available otherwise.
Now if I remember right portage gets the tarball untars it and compiles it.
I do know how to just d/l the tarball but is there any way that I can put different drivers into the kernel and xfree after decompressing before it gets compiled?

hope I didn't miss something really obvious,

Klaus