What to use as an external 100 Hz modulation source for an RF Signal Generator?
If you have got that HP 8656B, you shouldn't have any trouble to get a proper low freq signal generator to be connected to the mod input of HP. SDG805 is more than a valid one, if you can get it. Only remember to have its output amplitude so low that HP keeps all smoke inside. I would add an attenuator between them for safety.

Modulating with square wave sounds suspect. You should be well aware of actual available modulation specs in HP. Modulation with square wave in theory demands infinite bandwidth. Hopefully in your team exists one that understands the modulation mathematics.

User @Chris Stratton has inserted the following comment: 

Most uses for 433 MHz radios would expect OOK (=on-off keying) or FSK (=frequency shift keying) modulation by a square data signal. However, the normally utilized transmitters may have a narrower modulation bandwidth than the signal generator does (in order to avoid splatter), so it might take a profiled modulating waveform to accurately simulate with a signal generator the kinds of transmissions likely to be encountered

Thanks to Chris Stratton!