Note - it's not only impossible for a domestic compressor to get thread right pressure to fill a cylinder but it is not filtered for 'breathing air'
If you were to manage to fit an adaptor onto the fill nipple, and your cylinders current pressure is low enough then you could get some air into it
This would be 'dirty air' and at best risk your regulator or burst disks clogging up and failing, a burst disk could burst (which is good as that is a safety feature), your regulator would need a service before replacing the burst disk
The worst case though is contamination inside the cylinder, especially oil
Then everytime it gets filled to the correct pressure (EG 3000psi) it warms up and acting at high pressure can create a Diesel engine and have a flash explosion
You and anyone around you in danger of severe burns and death
This was not caused by oil (not by the user at least) but was a flash explosion when filling: