Swapping L15-30 with L21-30
You can't do that.  It'll fry your equipment.

What you have is 240V "wild leg delta" (orange phase). This is 240V delta with a neutral point inserted halfway between red and black phases.  It is 240V specifically, so it will be 120V from neutral to red and black hots, and 208V from neutral to orange hot.  This gives it compatibility with 120/240V split phase (residential) service.  The NEMA 15 doesn't use neutral, you are powering a 240V/3 phase machine which doesn't care about neutral.  

The NEMA 21 plug you want to install is for 208V "wye" 3-phase, with neutral in the middle and 120V from each phase to neutral.

The difference in phase-phase voltage (240V vs 208V) and the high difference between neutral and wild leg (208V vs 120V) is going to break whatever equipment you try to plug into it.   

You might be able to jumper the motor for 240, but you won't be able to jumper the auxiliary equipment for 208.  You may be better off adding a 120/240V transformer inside the equipment to power its 120V auxiliaries. 

Further, you can never bootleg neutral off another wire such as ground.  If the above hadn't been a problem, you would need to fish an actual, white or gray neutral wire through the conduit to be neutral.