It depends entirely on what that switch on the microphone switches when pushed. If it's only the 'keying-line', then a separate hand/foot switch should work just dandy. It that mic-switch also keys other things, then you're going to have problems if that hand/foor switch won't also key those 'other things'.
Can it be done? Sure, that hand/foot switch just has to do that 'other' switching too. If I'm not mistaken, that Heil hand switch is just a single contact switch, only switches one line. If you can find a multiple contact switch, put it into a hand-held thingy of some sort, it should work. Depending on how bad you need/want a separate hand switch, you can certainly work up a way of doing it. May not be all that 'simple', but it's certainly do-able.
- 'Doc