Another vote for Melodyne pitch-to-MIDI conversion with guitar, then dropping the MIDI notes down an octave and driving a virtual synth.
 
Another option is to simply record your guitar part, and use Melodyne or SONAR's transposition DSP as Zargg71 mentioned, but that much of a stretch will probably sound less than wonderful compared to using MIDI to drive something like SI-Bass.

The audio to MIDI via Melodyne is fine, as long as you play a really basic, rudimentary, clean part. Try adding in slides, slurs, or bends, and it will become an editing nightmare.
 
I'd just get a bass, or ask any one of the many bass players on this forum to record the part for you, it'll be better.

Anderton
 
Another option is to simply record your guitar part, and use Melodyne or SONAR's transposition DSP as Zargg71 mentioned...

I've gotten ok results using this method with SONAR's transposition process (at least to my ears).  I recorded a clean DI signal with my guitar, selected the wave file, then clicked Process - Transpose.  I then checked the "Transpose Audio" box, dropped the amount to -12, and chose the "Radius Solo (Bass)" type.  I then ran it through a bass amp sim.  The process was pretty quick and sounded pretty good to me.  The other suggestions made here may get better results, but I have not tried them.
 
If you're interested in hearing an example of what I described, here's a link to a quick little blues guitar video I did with the bass done this way:
http://www.dailymotion.com/video/x4gwm79_little-blues-guitar-jam-in-g_music
 
 
 

Wow... all excellent and timely suggestions from top to bottom---no pun intended.
 
Now if I could only program my brain to think like a bass player and get my less than nimble fingers to execute!
 
Thank you...

Zargg71
Hi. You could try to pitch shift the guitar down an octave. Do not know if it will sound ok or not.
All the best.

Does the TH3 Sonar version include the pitch shifter? If so that and a bass amp sim. I like the TH3 bassman, but I don't know if that's included in the Sonar version ...
 

Do you have any HW that will do it? I use a Roland VG-99 for bass most of the time.
 

rsinger

Zargg71
Hi. You could try to pitch shift the guitar down an octave. Do not know if it will sound ok or not.
All the best.

Does the TH3 Sonar version include the pitch shifter? If so that and a bass amp sim. I like the TH3 bassman, but I don't know if that's included in the Sonar version ...
 

You can just use Melodyne or Sonar's built in DSP Transpose process to downtune the guitar file. It would probably turn out better than a guitar sim pitch processor.

fjz
 
I've gotten ok results using this method with SONAR's transposition process (at least to my ears).  I recorded a clean DI signal with my guitar, selected the wave file, then clicked Process - Transpose.  I then checked the "Transpose Audio" box, dropped the amount to -12, and chose the "Radius Solo (Bass)" type.  I then ran it through a bass amp sim.  The process was pretty quick and sounded pretty good to me.  The other suggestions made here may get better results, but I have not tried them.
 
If you're interested in hearing an example of what I described, here's a link to a quick little blues guitar video I did with the bass done this way:
http://www.dailymotion.com/video/x4gwm79_little-blues-guitar-jam-in-g_music
 

I used to do the same thing until I purchased a used Squire P-Bass.  This guitar transposed down an octave does work.
 
PS - great blues man!
 

lowerfreq
Now if I could only program my brain to think like a bass player

Just remember that you're part of the rhythm section, not the melody section 

I would consider using an acoustic and just eq it bass heavy.