I've experimented it with a bit, but apparently it's not that easy: f2c seems to make all sorts of assumptions about variables, functions and expected output, many of which turn out to be wrong (e.g. by default, it interprets the statement "a = foo(1,2)" as if foo were a function, so it turns that into "a = foo_(&c__1, &c__2)" which is useless for me). To coax it into generating usable output I'd have to declare variables with their correct dimensions and sizes and so on, so in the end I'd have to replace the original non-trivial problem with several more, even more non-trivial problems.
Extracting the expression parsing part from the source might be an avenue worth exploring, but then there is this comment in the linked pdf:
"The program f2c is a horror, based on ancient code and hacked unmercifully. Users are only supposed to look at its C output, not at its appalling inner workings."