I was given a list of contacts from a different company to add to our companies address list.
It now happens that everyone from the service compartment doesn't have a personal address, but only a shared "service @ company.xx" address.
This shared address is the only way to contact them.
Can i somehow assign one address to multiple external contacts?
No, as Exchange enforces uniqueness requirements for email addresses, you can only have it assigned to a single recipient object.