Is there any hack known to enable BroodWar units (like medics, valkyries etc.) to be built during the original StarCraft campaigns?
You can't during the default campaign but there are modded campaigns out there that are built as brood war maps (common one to look for would be the coop mod) in those ones using the all technology cheat should enable brood war units.
Alternately you could extract the maps yourself and just load them up as a brood war map (using map settings).
Here are two guides (I haven't tried either of them but they look right):