I'm not sure, but i doubt you could successfully use your tank drain to feed your skimmer, and I'd be a little concerned about your tank flooding in the event the skimmer got clogged or something like that. One thing you could do is drill a hole in the sump wall, put in a bulkhead, and use an external pump. You could also run some plumbing up over the side of the sump, and back down below water level, then hook up an external pump. You'd have to prime it to get it started, but as long as the plumbing in the sump stays submerged, it should hold a siphon if/when you shut off the skimmer pump or have a power outage.

I don't know anything about the skimmer you're using, but most skimmers will work bettter with a better pump, and rios are pretty much among the worst. Mag pumps can be used external and are better than rios, and not much more expensive.