If the source code can not get the simple options there.
There is the option of disassembly, build your dll, but this operation is so complex that judging from your question it makes no sense to undertake.
If the program is simple, easier to order as a freelancer and write from scratch, obtaining and software and source code for the future.
ashlee_Stiedemann52 answered on July 2nd 19 at 17:39
If no source - it's much easier to write from scratch.