Okay... so if you can see the MIDI data on the track you should be able to change the track to output to a soft synth on your computer. I'd say clone the track and mess around with the input/outputs on the new one to see what you can get going. Sorry I can't be clearer but as I said I am not used to dealing with external MIDI hardware.

In the future you'll probably be much better off just writing your MIDI parts right inside Sonar using the Step Sequencer or Piano Roll View.

Hope you get it figured out. 

Sure, I knew that I would apply it to some synth or some kind of patch, but I didn't which would be the best option.

I like the SI Bass very well.  Also, you can download the free Kontakt 5 Player from Native Instruments and get quite a few free instruments from them, including a good electric bass and acoustic bass.

Well as I said, the Zeta Alembic Bass patch is a pretty real sounding bass tone that's good for setting a foundation. Then clone the track and poke around the other Zeta bass patches (or any of the synths really but the Zeta bass sounds are great IMO) and then blend them. Most of the included synths have a Bass folder for patches though so go hunting. Tons of great stuff. Also check out the Garritan Pocket Orchestra stuff or the DSF strings (if you qualified for that free pack) for orchestral cellos, double basses and various configurations thereof. Seriously... you can get lost for hours in all those patches. Have fun and enjoy. ;-)

I've had good fun bringing in a midi track from the DR880.
The DR880 has its own sounds that have lots of different bass sounds too but its audio as you say.
But I do prefer to bring in a bass midi track as there are so many synths to run the midi thru.
Also a copy of the midi track can add depth and tone using a second synth setting.