Acceptable to add knockout to PVC conduit body?
I believe it would violate code even if it had a stamped volume. The modification would weaken the structure. A stamped conduit body can be used for splices, but conduit bodies are separate from J boxes that are allowed to be modified.
I would probably change over to a “anybody” conduit body. These are configurable straight, left, right, T , and an extra port can provide what you want. I used to carry a couple of these in 1/2, 3/4 &amp; 1”. You don’t have to have all of them if you want to save space.
There are also threaded covers available through Hubble. I think I have picked them up at Home Depot as well, similar to the anybody conduit body.