The only way is to export individual tracks and import it one by one to Cubasis.
Cubasis also supports midi files by the way so you can import your midi data and then work with iOS synths.
I use different software to start the track it can be Cubase, Reason, Maschine or Logic. To continue working in Cubasis sometimes I just mixdown (all tracks together) and continue adding instruments in Cubasis (with Audiobus and other ipad apps). It is pretty easy and fast.