Can a drywall screwdriver be used as a regular power screwdriver?
Yes, I've done it for several years.  However, drywall screwdrivers are considerably more expensive than regular power screwdrivers (battery or corded) because of the clutch that allows them to engage and disengage the torque upon press.  Once you get the hang of setting drywall screws and develop the sense of how deep to go without the guard (if you haven't already), you will be able to set them with a regular power drill and a Phillips bit.  It is not that hard.  Just in case you were contemplating a purchase of a drywall one, I would say you don't need it and should go for the simpler and cheaper solution of just using a regular power screwdriver or even just a drill with a Phillips bit.  I hope this makes sense.