I have inserted Toontrack EZDrummer 2 as a simple instrument track. I cannot figure out how to assign the MIDI output to a custom drum map. I have already made drum maps for many of the EZX libraries. But, I can only use them when I have separate midi track and audio track set up. 
 
When I go into piano roll I cannot use the drum pane. 

At the bottom of the Track Inspector for an Instrument track you'll find MIDI and Audio tabs. If you select the MIDI tab, you'll have access to the Output assignment for the MIDI where you can select your drum map.
 
Or you can just temporarily split the Instrument track, and use the Output widget in the MIDI track.
 
That said, Simple Instruments tracks aren't often used with drum synths because they only let you use the Master bus output of the synth so you have to do all the mixing in the synth GUI, and can't use SONAR FX or automation on individual drum sounds.

Cool. I didn't see the AUDIO or MIDI tabs at the bottom of the track inspector. 
 
I used to route all of the EZDrummer outputs to individual Sonar audio tracks.
 
With EZDrummer 2 I am finding that the preset mixes in are working well enough that I've been just doing minor tweaks inside the plugin's mixer - if at all. 

Can you "un-split" a previously split instrument track? 
 

yes

Select both tracks, right-click and choose Make Instrument Track.

No selection necessary, right-click on either the MIDI or audio track and select the option. The option is smart enough to know the other track which completes the instrument track.

Sweet. Good information. I have been neglecting instrument tracks completely. This will help tidy up my tracks a lot.