Have you tried the 'Search' button on this site?
It's definitely been discussed before, but I don't think the results were too positive.

The trouble with automatic translation between any languages is that you still have to check the results very carefully by hand to ensure that the result is doing what you really want - this takes about as much work as doing the conversion "manually" in the first place!

A previous client abandoned the idea of automatically converting their legacy Pascal code to 'C' for this very reason.

If your PL/M code works, it's probably best to leave it alone. Just convert it on a module-by-module basis when you actually need to do some work on a module.
There shouldn't be a problem in calling between 'C' and PL/M (Keil supports it).

BTW: There is a story, probably apocryphal, of someone who tried feeding the English saying "out of sight, out of mind" through an automatic translator and then automatically translating the result back to English - it came back as "invisible idiot!"