Verified answer

You could fill your HPA tank or bulk tank to 120 psi with your 120 psi compressor. Given that the input pressure on the average paintball gun is around 800 psi, you might get enough air to fire one shot per tank that way...but I doubt it.

A compressor designed to PROPERLY fill your HPA tanks costs $549. I've included a link to it. Note that the Shoebox is not stand-alone, it's powered by a shop air compressor; Your 120 psi compressor may be able to power it, but you might need to install a regulator to step down the pressure a little.

Added to comment on other comment: While it's true most compressors that can fill a paintball tank are expensive, the shoebox isn't expensive. Not compared to what a standalone compressor that can fill a 3000 or 4500 psi tank costs, anyway. Standalone compressors that can do that start around $4,000 for a cheap one.

Can it fill it?... sure with the proper adapters for an hpa tank then yes it can fill it with the 120psi the compressor has at its disposal. As the other person stated the average paintball gun works at 800psi so this will not be efficient at alll. As you have just stated your tank holds 3000psi so you will not reach anywhere near that and so you are forced with getting an expensive stage compressor (each stage compresses the air more until its final pressure). My advice if you really want to fill from home? buy a scuba tank, they are relatively expensive but you can take this to a scuba shop, fill it for like 4-5 bucks and then use that tank (with a fill station regulator of course) and fill your tank multiple times. This is a less expensive alternative to the shoebox or any other High pressure air compressor but its still pretty expensive so weigh your options and see if this is in your budget or even necessary to you, i hope i helped and good luck!