You can do that, but building your own cable would be more complicated; a standard microphone cable doesn't have all of the necessary connector pins, and soldering your own connector would be tricky, if you can even find just the connector.
If you make a rear accessory port USB programming cable, just make it long enough to reach out from behind the radio; leave it connected and simply plug in whenever you need to program the radio.
if you're determined to do it through the mike port, you would have to buy a cable on eBay, which will cost you anywhere from $30 to $50 or more.