That's quite a big question, one problem that stands out is the odd shape of the GT540's screen, it doesn't stop as such where the screen stops, it stops before the real buttons, that would suggest to me that you'd have to have a custom screen and digitizer with those resistive buttons we have at the bottom (MENU, BACK) changed to capacitive, a custom screen would probably cost more than the phone itself. But if you do decide to waste a load of money outfitting a phone instead of buying a new one, on top of that you'd have to be able to program drivers for the display to work and you would have to modify your software that your phone runs top work with multi-touch.
So in short, really, no.
But if we had a load of developers (which we don't) what could be designed is a counter-point which isn't multi-touch in the traditional sense and is a software work-around, but still a bit of a waste of time.
LG Optimus GT540