Shocks are not charged with air; they are charged with nitrogen.
There is a good reason. When pressurized to 300-ish psi, and loaded with finely dispersed oil mist, and rapidly pressurized, as happens in a mountain bike shock, the air gets confused about whether it's in a shock absorber, or in the combustion chamber of a Diesel engine. ... and every so often, decides to ignite in a Diesel combustion event, which will cause a shock absorber to explode, and attract trial lawyers.
For the same reason, hydraulic accumulators are charged with nitrogen, not air.
I have not found documentation of the event, but I have heard an assertion that a loss of life event at the West Milton, NY site, where submarine reactors were developed in the Fifties, was due to the explosion of a hydraulic accumulator charged with air instead of nitrogen. I assume that any reports were classified just because they were associated with the Navy Nuke program, even though the event itself had nothing to do with nuclear stuff.
So if you are going to add gas to a shock, it should be oxygen-free. There are several ways to do that; have fun exploring that issue.
Additionally, I'm not sure how changing the gas pressure will have a noticeable effect on the shock's performance, unless there is some magic portion of the hydraulic valving that responds to the pressure environment. I suspect shocks were originally pressurized in order to shrink the size of any gas bubbles in the shock oil, and hence reduce 'fade'. You may find appropriate papers at sae.org. Last time I looked, the index to SAE papers wouldn't even fit on one CD. Ask if your school library has an account with SAE.
Third, you may not need a motor. There exist self-leveling shocks that pump oil in and out of the core of a shock absorber in such a way that the shock tries to maintain a fixed length, using a piston separate from the shock piston itself, driven solely by the motion of the shock ends relative to one another. You may be able to adapt something like that to your purposes. Again, sae.org should be a good source of information, if you look hard enough.
Welcome to engineering.
Mike Halloran
Pembroke Pines, FL, USA