I have an ONq legrand cabinet with an on-q telephone panel inside. the phone lines in the can have telephone plug connectors on them and are plugged into the telephone panel. I have 4 phone line extensions that have DSL filters plugged in at the wall outlet. Couldn't I just unplug those filters at the wall outlet and plug them back in, inside the cabinet, between the phone lines and the phone panel?

Yes, you can move the DSL filters into the on-q enclosure and they will work the same way. I always try to put the DSL filters in the can, just because it looks cleaner than having them hanging out of the outlet.