I dont think it is working that way. You can export it to step, and you can provide the a mechanical layer gerber/drawing with dimensions. Otherwise,"make a screenshot, draw the lines in paint". Altium is not really that kind of CAD designer, the 3D is there for the electronics engineer's not the mechanics engineer.
STEP has a lot more information anyway. Send it to them, ask them to send back their proposal (in step, not stupid 2d!). Not the other way around.