Hi Daniel -
If I understand correctly, you're trying to set up a monitor [a.k.a. cue or foldback] mix for your headphones [fed from one of the Aux Outs on the Behringer] that includes additional Logic tracks [in addition to the live signals feeding into Logic] + the Logic click..? If that's not it, you'll need to explain in more detail what you're looking to do..
But assuming the above is correct, it would seem that you'd have to set up the mix for one of the Behringer Aux Outs in the software utility mixer that comes with the Behringer. Then I guess you'd set up a cue mix on a row of Sends in Logic and route that to one of the Behringer Aux Outs, and do the same with the Logic click, route it to the same Behringer Aux Out. Assuming that those Aux Outs function as DAW interface outputs as well as Sends for the incoming [analog] signals, that might be possible [see below]..
Now, I can't say how that would be done in the Behringer software utility mixer, or even if that utility allows for that -- the signal flow diagram in the manual in unclear as to exactly what options are available regarding audio from the DAW/USB connection. I guess you'd have to go to the Routing menu in the software utility and see what signal[s] from the USB connection can be routed to the Behringer Aux Outs -- if it's possible to do what you want, it looks like that'd be the place you'd do it. If that is possible, then I'd assume you'd see those Auxes in Logic's Interface Output menus as additional outs, and you'd route your Logic cue mix [as above] and click to the one you want.
The XR18 manual seems to imply that that might be do-able [p.23], but I'd have to have the XR18 and software utility in front of me to be any more specific..