I want to pump an under-slab reservoir for a bathroom that is below grade. I need to connect a 2 inch wye into the main stack which is near-horizontal 4 inch ABS. There is a clean-out a few inches uphill from a toilet. Can I just extend the clean-out and add the wye? Alternatively, I would have to cut the main stack and how do I insert the wye into a stack that will not allow movement because of existing junctions.

If you're asking if you can remove the clean out cover and use the threaded part to take a wye off from the answer is no you can't.
What you can do is take a 4X2 wye,( try to get one with a bell on one end and plain end on the other. If all you can find have a bell on each end glue a 5" piece of 4" ABS into the bottom hub. Measure the fitting from the bottom of the hub to the end of the short piece and add 1/4". That will be your cut. I would support the 4" main with perverted iron strapping about 6" on either side of the cut so the main doesn't sag or fall. Masker your cut. You may now glue in the hub end, giving the 2"branch some pitch, and secure the bald end with a 4" No-Hub Band.(see image) You may now take off from the 2" branch. Don't forget, if you're adding a fixture it MUST be vented.Good luck, Tom