Imvu has been heading towards the use of other 3d modeling programs so not just limited to 3d Max, Sketchup and Blender. But for programs such as Zbrush, Marvelous Designer, Maya or any other that has ability to export FBX format, as a means to open up to more meshers.

Marvelous Designer seems to be what most are turning to for clothing and has some nice features.

Found some tuts:

https://www.youtube.com/user/scop880/videos

these are for 3dmax or at least the shoes one im watching lol

Marvelous Designer and Zbrush could already have been in your personal pipeline prior to fbx support.
They're both more so middleware software in this particular context (creating IMVU assets) in the sense neither truly replaces your traditional modeling package, though zbrush is certainly inching into that territory. Both of these programs help you create very complex details, albeit in very different ways, but for use on IMVU you'd really never want to go directly from either straight to importing to IMVU because A) ideally you want to end up with a fairly lowpoly diffuse-only asset, which is not something either of these programs is really meant to leave you with (not to say it can't be done) and B) there's still some prep work to do before going from FBX -> IMVU that, at least as far as I'm aware, you can't do in either software.

I'm personally mostly looking forward to this change just to get away from some of 3DS max's worse-for-wear toolsets that didn't receive a whole lot of love after Autodesk acquired Maya.

I've been looking into Marvelous Designer because they mostly cater to clothes creating. I plan to check it out soon after I toy around with 3DS Max and their Garment Maker plug-in.