Bipolar power from unipolar supply, but accounting for DC offset
You can probably use a virtual ground circuit with the virtual ground connected to the sleeve of the synth CV input. But it will only work if the pedal power supply is floating. If it's not floating, it would indeed be a short and something would probably burst into flames. But a wall wart supply is almost certainly either a transformer or switching supply that in either case are probably floating. So you can assign ground to be whatever you want (in this case the synth ground). But you need to test to make sure by creating the virtual ground circuit, put a 1M resistor between the sleeve and virtual ground and then put a meter across the resistor to see if there is a voltage across it. If there is, the supply is not floating. There might be a small voltage indicating a small current (1V across 1M would be 1uA) which is fine. But hopefully the voltage will be really small like 50 mV or so. If that is the case, you can connect the virtual ground and sleeve of the CV in. But you might want to use a small resistor like 100R just in case someone plugs in a non-floating supply so that it doesn't burst into flames immediately. Or maybe use a 10 mA fuse.

Note that I very much doubt the CV uses 10's of mA. 10 mA is enough current to light an LED pretty bright. The currents involved are probably 10 uA and not 10 mA.

Note that you will need to check the pedal CV for noise (while connected to the synth) using an oscilloscope or (PC audio interface if you don't have a scope). Whenever you connect two devices with different supplies, there is great potential for noise. But using the synth ground as the ground for the expression pedal might be just the fix for that sort of noise problem.