ok what i have done so far:
I set the navmesh to rebuild at runtime and added a navmodifier to my actor. I set the navmodifier type to obstacle instead of null. Now my units path to the building almost correctly and path around the building when it is not targeted. My only problem are the units themselves now. I tried to add a navmodifier to the units and set them to obstacle too. But now the units get stuck in their own navmodifier and basicly move and turn on the spot. (Looks like a weird breakdance.) I will post updates here once ive found a solution.