Precisely at stake, the M2 is not visible while in Noggit they are.
For WMO, I found a patch that someone had created to add these objects.
1) open wow cata with wowmodelviewer
2) select you model (example gilneas tree)
3) export to 3ds
4) open MDLvis target ONE vertice
5) Edit> Paste from the file (open you 3ds tree)
6) - 3ds converted to mdl in MDlvis
7) convert mdl to m2 (Mjollna tutor)
add collision to you model (yeah.. Mjollna's collision adder)
9) add in noggit ...
10) profit. - cata doodads in wotlk 3.3.5!
this screen was created in 3.3.5 (by zim4ik) cata gilneas convert