I want to add a sub panel as part of turning the basement into finished space. The Main panel is in the garage. From the Main, a 220 line was ran through conduit to the basement and out to the AC.

My question. Can I take the 220 line that goes through the basement and to the AC and make it the feed to the sub-panel. Then from the sub-panel connect to the AC? I would also add to the sub-panel outlets, lighting and frig for the basement.

Since the AC feeder is probably sized just right for the AC, any added load could only be used when the AC is not running, and vice versa. Is this practical?

You could do that by rewiring but what is the AWG supplying the AC and what is the breaker capacity? It is most likely dedicated to the AC and will not have the capacity to run the AC, outlets, lights and refrigerator.