"converts PLM-51 code to C code"

PLM-51 is a language specifically designed for the 8051; 'C' is not. Therefore, you will also have to define which particular set of 8051-specific 'C' extensions you want!

This has been discussed before - try a 'Search' (try both "PLM" and "PL/M", etc)

Automatic language translation is fraught with pitfalls - especially with a free translator, which probably gives you no support and no guarantees.
It is probably better to just create a well-defined interface between your existing PL/M-51 and your 'C', and use inter-language calls (both Keil & Raisonance have support for this).
Only rewrite the PL/M-51 functions into 'C' as and when you really have to!