Step import
 

I created my model in inventor, saved as .step and imported in designmodeller. While the thingy was a 1 volumebody in inventor it turned out to be consisting of many parts (4) in designmodeller. How can I join them since so far I'm concerned the body will not be recognized as one for later calculations!?

You can either use "Body Operation" or "Merge" or "Connect" based on your geometry.
You can always create interface between to surfaces.