looking for a solution to migrate from bloghoster to WPMU

Hello fellow WPMU'ers since five years i use Bloghoster, since that system is not maintained for quite a while i would switch to WPMU, but like always there are some issues.

1 - i have over 10.000 posts in BH.
2 - I have over 600 Users.
3 - i have 20 GB of data (images and other files)

So i need to have some help, is there any conversion script out there to do the migration job for me.

// Peter