This all depends on the project.... If the code has been written with portability in mind, there will be little conflict. If the Pic18 code was written in C18 then you need to get a copy of C32....
The assembly part will cause problems. Personally I would convert the asm to C first as the pic32 is tons faster so native C will also run faster.....
I use MikroC for pic32 and I also use C18 and XC8.. I can port C programs to MikroC with very little effort. I have also ported C18 to C32 with very little problems...
Asm is very different, so conversion to C first will be the quickest way!! Unless, of course, you are well versed in assembler!!