There is actually more to consider if I understand what you want to do. Having the skimmer cup drain into a drain is a bad idea in case something causes the skimmer to go nuts. By nuts I mean skim wet, over skim, over foam, or anything else similar. You could end up draining many gallons of water which in turn will cause ATO to trigger filling more. Salinity will change risk 1. Risk 2 is that your ato is out, skimmer continues, and now sump is lower and now you have a loss of water causing pump failures. If not that then temperature changes.
Set up a larger container, add a float switch, and have it turn off when triggered. I would never recommend draining a skimmer cup into a drain. Ever. Just like I would never recommend using dry rock