Theres a difference in a supplied air pump used for breathing air and getting breathing air off from a regular air compressor or the system your using.
There is some risk involved in using supplied air, and as far as rigging up your own system from the pump your talking about, you increase the risk.
To many things can go wrong, I wouldn't attempt it.
I worked close to 30 years with supplied air, scba, negative and positive type respirators.
We trained people on how to use them and we also did all the maintenance on the systems.
Supplied air is not something you want to experiment with, just don't go there.