The way to do this is to use the auxiliaries. There's 5 aux, of which you can route aouch... don't know it all by memory. Anyway, you can route some of the auxilieries to some outputs. From there you go to your external effects, and via one of the inputs you get it back it...
Kinda like you'd do it on a regular mixer, with the only difference that you don't have a dedicated aux in.
2 or 4 of the aux are by default routed to the internal effects boards, but you can change that. Look in your manual for more details about that.