another tank is a possibility. if you have the room for it, it would def work...
or another option would be drill a hole in your sump for a bulk head and plumb the return pump like that...
just a suggestion to you. to make things alot easier. i would consider removing that sump. get something like a 20L or so and make this your sump. with doing this, you'll have a chamber for your intake with the skimmer, then baffle into a fuge (deep sand bed,live rock, live rubble rock, macro), then baffle into the return chamber where your pump will be held....
seems like alot but much easier, a little DIY project, more hands on, you can design the sump any way you want, and you'll get rid of those bio-balls...