Can I intentionally overfill my hydraulic brakes to adjust contact point? I have a bike with SRAM Apex hydraulic disc brakes. They do not have contact point adjustment, but I want the lever to firm up earlier in its travel. Can I just overfill my brakes with fluid to adjust contact point?

If so, what would be the best way to do it? I do not want to overdo it and be unable to fit the pads and rotor back in.
No - that would lightly pressurise the system all the time.

From https://www.sram.com/avid/setup-guides/hydraulic-brake-setup  


  Note: The Contact-Point Adjustment DOES NOT move the pads.


Instead you want to try the reach adjust, and try tweaking the caliper mounting bolts so the caliper is aligned how you want it.