I have a HIWATT G50MR and have thought about buying a stack, but I was wondering if I could just buy a amplifier head and connect it to the top of my existing amp. Is this possible?
You could certainly use a different head and just wire its output straight to the speaker, but that would mean you just have a redundant amp sitting there. So effectively you would have a stack, but not in a very efficient way. To do this you would just remove the existing wires from the HiWatt amp to the speaker and run them from the new amp to the speaker instead.
Or you could use a preamp before the existing HiWatt amp if you are wanting to get a different sound (although the appeal of the HiWatt is its very British rock sound). This would just plug in to the signal path, so no changing of wires needed.
Both seem a bit pointless though. If you want a stack I would just bite the bullet and get one.
In addition to rewiring the amp, as Dr Mayhem noted, there's another general option. A guitar amp consists of two separate amps a pre and a power amp. In general, the pre amp is used for your tone and the power amp is used for volume (yes, yes, they both affect each, we're talking pretty general here). If your amp has an effects loop, this loop allows you to insert things (usually effects) between the pre and power amp. A normal head includes a pre and power amp, but you can buy the two separately and then combine them. If you have a pre amp, you can plug the output in to the effects loop. This would allow you to use the new pre amp and the existing power amp and speakers. If you get a head with both a pre and power amp, but it has an effects loop too, you could go out from the head and in to the combo and use the different power amp with the included speakers.