If with "a small part" you mean a driver that you wants to let nitdroid use it you have two ways to do that:
1) compile inside the kernel
2) compile that driver as a module (and load it with the commands insmod)
for both choices you can find a lot of tutorials by googling..for example, to compile a driver as a module here there is a little guide:
http://android-jotting.blogspot.it/2...driver-as.html