Well, a lot of code can certainly be borrowed, but almost none of it
will be a drop-in replacement.
What I recommend you do is not worry about doing a full port and instead
just make a big dent by getting the boot sequence to bootstrap into a
64-bit built DragonFly kernel. Even if you can't get it much past that
it would be progress. Just getting it to boot that far is probably
a big job unto itself.