If the A/C shop near you won't work on an existing R-12 system it's likely because they are not qualified/certified. As stated above, you need a 609 certification and more to handle this refrigerant.
If this is the ONLY shop you care to deal with and you already have the R-12 and the little do-it-yourselfer hose, then I'd say you may want to give it a shot yourself. If the system is bone dry, determine how much the unit holds. Check SEARCH facility here. You may even find capacity charts at http://www.aircondition.com.
If it's not bone dry, but just needs replenishment, start out with a can. As stated above, gauges are better, but if all you have is the diyer hose, you're options are limited. Watch the sight glass AND have a thermometer in the center vent. Crank up the air volume. If the system has some charge in it, let it cool a bit before you start charging. I've always wrapped a hot cloth(dip in hot water 1st) around the can
before I open the valve. This helps the refrigerant to flow better.
You may as well accept the fact that whether you do it, or someone else does it, you're most likely going to have some R-12 left over and as you know, it will leak out of the can.