Every company is a little different, but in my experience the only way you can get a positive space ticket is if the company gives you one. For example, at United they give you a positive space ticket to travel to the training center for a PC check. Or if the company needs to get you to another city to cover a flight.
There are other speacial cases, like a death in the family and you get one from your supervisor. When I flew at the commuter level, the gate agents could issue a positive space ticket but they had to give the company a good reason why they did it.
Remember, when you get a positive space non-rev ticket, that means you will bump a revenue PAX, so the company will only do it if they really have to. hope that answers your question.