air conditioner

My daughter has an 01 SL2 which has an air conditioner leak, I believe.
I've added the "freon" a couple of times over the past year. I think she
let it get too low now to add any more because now the compressor doesn't
kick on and therefore doesn't suck in the new stuff. I think before when it
was low, the compressor would kick on for a few seconds, then go off, but
after the new "freon" was sucked in, it would stay on longer, etc. Is there
a way to charge the system somehow without going to a dealer who does
whatever magic needs to be done to get the compressor to come on when the
system is too low.

thanks