Let me guess. You drive a Honda, right??? I slept at Holiday Inn.
Not sure if I can help you, but you're probably gonna need an overflox bow or have to drill a hole in the back of the tank for the water line to the sump. The a pump returns the water from the sump back into the main - assuming the sump is situated lower than the main tank. Check our/google on aquarium overflow boxes.
I'm sure one of these smarter folks will be along to help, but you sure you don't want to just get a new 20g and use the existing 10g for the sump?
"Life may not be the party we hoped for but while we are here we might as well dance!"