I'm assuming you don't have the Onan generator in the RV, just the prep package for a generator.
It should be entirely possible to wire in a connection for the Honda generator to the prep wiring. Fuel connections and physically installing the generator in the space are likely to be rather more difficult, and might be altogether impractical or impossible.
Note that the Honda EB6500 and EM6500 generators (in all their subvariants) are open-frame, contractor style generators and as such fairly loud. The EU7000IS is a quiet inverter generator, and would be much preferred, particularly if you're camping in proximity to other people.
If you have the Onan generator installed, paralleling it with a Honda generator won't work.