Current house came with a whole house fan (not an attic fan, but the
kind in a hallway ceiling that sucks air into the attic). I was happy to
see that because I really liked the one in my old house. However the one
in the old house had a variable speed dial on it, and the new house's
fan has just one speed - high and loud with a simple off/on switch.

Is the variable speed a function of the fan itself or can that be added
at the switch? I'd definitely be having an electrician do it, but I'm
just trying to figure out if it's even doable, or if I'd end up needing
an entire new fan. And if so, would replacing an existing fan and switch
be difficult (ie cost me an arm and a leg)?