In the root.epk is for each race and gender a .msm file and you have to change the coalisation typ but with that you can only run through trees/mobs but not through buildings, rocks, bridges etc.
I´m not sure but i think you have to change the coalisation typ from 3 to 1
Code:
Group AttachingData01
{
AttachingDataType 1
isAttaching 0
AttachingModelIndex 0
AttachingBoneName "Bip01"
CollisionType 3
SphereDataCount 1
Group SphereData00
{
Radius 70.000000
Position 0.000000 0.000000 100.000000
}
}