Not sure if anyone has tried this. I have no experience creating user tracks so don't even know if its possible. Could you take a cover song multi track file and create a user track from that in the style of that song to use in band in box.

The multi tracks are wave files so I would assume that if you created the original song template in BIAB and assuming the multi track file is timed correctly you could record this as a user track.

If possible users could create not only user tracks but user style sets. That would be cool.

The answer is yes, sort of. First realize UserTracks do not work with 3/4 time or holds at this point in time so if the multitrack song has holds or is in 3/4 time you likely would not achieve good results.

Now, let's say neither of those restraints are in the song then the answer is yes but you may not get the results you expect. BiaB and RealBand use pitch and time shifting technology to create notes that don't exist in the original audio material. Sometimes the original audio material morphs well but sometimes it don't. All of it may morph well except for a note or two. You just don't know until you try. That's part of the fun in creating UserTracks. Generally speaking audio material containing whole, half or quarter note audio will morph better than audio with eighth, sixteenth or smaller notes.

If you have a multitrack that contains predominantly audio built around the chords of G, C and D for instance it may not give you the results you expect when used in a song having a E, A and B chord progression.

If you are wanting access to specific riffs you can isolate the bars with the riff and import the bars as a loop. The same technology is in use its just that you are isolating the bars instead of BiaB or RealBand and you are providing pitch and tempo information while making the audio a loop instead of BiaB or RealBand using a SGU file as a lookup chart.

Ok so excuse my ignorance in this matter but how does PG Music get real tracks to transpose and time shift so well? Do they use a different process than we do in making user tracks or do they just include real tracks that they have tested and beat to death to make sure there are as few artifacts as possible?

Yes to what Jim said.
I requested MultiTrack Drums for BB/RB, in BB you can have a MultiTrack Drum Mixer window to get the drums how you like and in RB they just split into separate tracks.
There are some multitrack drum here http://www.realband.org/drums.html you can just drag n drop the section/fills into RB. You could do the same as Jim said with loop sections/chords. You could use Audacity or REAPER if you have it to make them up.

RealTracks are basically the same idea as UserTracks (only bug at the moment stereo tracks generate in mono (let alone 5 channels) but should be fixed in next version), Reatracks just have a lot more recorded time than most currently available UserTracks as PG only supplied basic UT templates, but there's nothing stopping anyone from making longer templates.
If you open a current rhythm RealtTrack wma/wav one at a time in the Audio Chord Wizard (ACW) you can create some templates from these. There's lots of playing, lots of keys n chords, this is what makes them good. You CAN'T have a good UT with a 1 minute template, even the 10,20,30 minute, there's way more time in RealTrack recordings that's all.

Don't know but I bet a lot of people would like to know the answer to that same question.

Truthfully, I don't think it is fair to compare RealTracks and UserTracks. RealTracks are entering their sixth or seventh year of development while UserTracks are about two years old. PG Music has much more experience developing RealTracks and identifying what techniques work or don't work than any users. Also, don't forget that PG Music knows the rules and restraints because they invented both features! But on the other hand as people experiment, learn and share information the depth of knowledge about UserTracks will rise. The pitch and time shifting software tools get better every year so what may not seem possible now may be more practical sometime in the future.

The nice thing about UserTracks is there are no UserTracks experts! Experiment, learn and share your experience with us!