Well, my short answer is that I don't know but I'll speculate with you if you like. Not likely in my opinion. Engine rear main seal has no pump pressure against it to enable any transfer. Additionally you would also need a coincedental path to open in the converter to allow fluid ingress and that is pressurized so you would think that any fluid exchange would go the other way.
If you are running a red transmission hydraulic fluid and it goes dark like oil, it could be from slippage and heat. That doesn't explain though why you are getting a slight fluid level increase. Does the increase in trans fluid level coincide with a decrease in engine oil level? Are you running a trans cooler and, if so, is it engine mounted in a housing with the engine oil cooler?