Actually, not all is lost here. You cannot legally convert your 93 into a *new* machinegun, but you could certainly set it up with a registered HK autosear. When people saw the writing on the wall back in 86, they quickly registered autosears and even full auto trigger packs as registered machineguns. You can legally purchase one of these and put it on your HK series rifle.
Current price on a registered autosear is about $3500-4000, depending upon where you find it. The HK family (91/93/94/SP89) guns are selling for around 2200 and up. So, you could easily have an HK machinegun for 6200 or so. Considering the fact that you've already got the 93, you're looking at the price of the sear and the paperwork.
One advantage of a registered sear is that you can legally move it from gun to gun. If you happen to have a 91 and a 93 you'll have two machineguns! Pretty cool stuff.
Registered sears are pretty common -- other than the price, I don't think you'll have any problem finding one already installed in a clip on lower receiver that replaces the trigger pack you already have. You'll also need to replace the semi bolt carrier with a full auto bolt carrier.
Hope this helps...
danbrew :->