If you play guitar then just record the bassline you want on guitar (DI, totally clean). Sonar has "ARA" audio to MIDI conversion that uses the included Melodyne to detect the notes and convert them to MIDI. I am not sure which versions of Sonar support the ARA conversion (SPlat of course does).
All you have to do is create an empty MIDI track then drag the audio clip of your guitar part INTO the MIDI clip. It will automatically convert the notes to MIDI. I recommend holding Ctrl + Shift as you drag the clip into the MIDI track. The Ctrl binding COPIES the audio clip so you still have the original in its original track (not sure if necessary for drop/drag MIDI conversion) and the Shift key keep the clip lined up to the time grid as you drag it downward.
The detection isn't perfect though and the free version of Melodyne does not support "polyphonic" conversion. You can upgrade it though for chord detection (pretty cool, ya?).
That means only play one note at a time (it's a bass part so usually that's fine. Play it as clean and tight as possible (with no external effects that can garble the signal... FX Rack/Bin or Prochannel effects don't count because they are added to the dry signal/clip after the fact... not printed to the clip).
To clean up the signal even more before the conversion (for more accuracy) you can use EQing, compressor/limiters, transient shapers, etc to make that dry signal as clear and distinct as possible. Only do that if you NEED to. As in if the resulting MIDI has all sorts of incorrectly detected notes, doubled notes, missing notes, etc. You don't even have to worry about whether it sounds "good". It just needs to have each note clearly defined in a way the attack and pitch of each note is clear enough for Melodyne to do its thing.
Reason being that any bad detection means extra work to clean up the resulting MIDI file... and you WILL have to go through the MIDI file to correct these types of things. It can be very tedious so the cleaner the audio track the less work you have to do.
Once you get your MIDI file converted and cleaned up you can go completely apenoodles with. Quantize it, humanize it, arpeggiate it, feed through any synth (or multiple synths) you want, mangle it, whatever.
If you want a nice bass sound I used to use the "Picked Alembic" sound in Zeta+2 (might be in the classic banks... if not you probably don't have). Studio Instruments Bass synth is actually REALLY good and realistic if you want just a straight bass sound and it's included.
Me I picked up the Anderton Gibson EB5 sample pack for DimensionPro a while back and it's solid. Just sound great.
BUT explore all the patches in all your various synths. Lots of good bass sounds included in Sonar.
Also keep in mind that you can also run a simple bass sound (like that Studio Instruments Bass I mentioned) through the include amp simulators or other effects. Essentially your MIDI just turns into a straight up dry bass sound and then you treat it like a live, mic, amp'd bass player/rig/sound.
Make sense?
Cheers.