Hi!
No, you don't need to recompile. If you read the user manual carefully you will see that you can provide a so called FORTRAN FILE as input. The aim of this FORTRAN FILE is that all the subroutines contained in this file (you can add as many modified subroutines as you want in it, one after the other) will be compiled automatically at the beginning of the computation so that your changes are taken into account WITHOUT modifiying the source code itself and recompiling it. So if you did modify the original prosou.f file, you should not do that.
All you need to do, in your case, is to take your modified prosou.f in a file that you can rename "my_fortran_file.f" if you want, and add the keyword FORTRAN FILE = 'my_fortran_file.f' in your steering file. my_fortran_file.f can be located in the same folder than all the other model files.
Good luck!
PL