Can I turn a 20 deg synthetic into a winter over bag?

This might work if the other critical elements of the system – the sleep pad(s) and shelter – are also up to the task.

Some 15 years ago I used a +20 synthetic MH sleeping bag combined with a synthetic Wild Things parka and MH Chugach pants (also synthetic… you see the pattern) with a 3/8" Evazote CCF pad (the ubiquitous yellow ones) with a thin (1.5") Thermarest self-inflator 3/4 length and slept comfortably down to -20F. I'm a warm sleeper and didn't need booties. This was also inside a winter tent, either a Bibler Eldorado or a MH Trango 2. Of course, with full winter gear and consumables my pack weighed 50-55lbs, and it was the lightest one in our group.

The only way to know for sure if all the bits are going to work together is to do a quick local trip and go a short distance into the woods (or backyard) when it's cold and give it a field test.