Hi Marcus,
It's the old Diagram Interchange problem again. Basic XMI does not include a standardized format for diagrams, but it is extensible. Meaning that all tool vendors are free to add diagram data to their extended version of XMI, which their own tool can then read but no one else's can. In short, using plain XMI diagrams cannot be migrated between tools.
This shortcoming of XMI was addressed years ago in the supplemental Diagram Interchange standard, which EA supports. But that will only help you if StarUML does too.
Failing that, you're looking at a custom translation tool. I'm not aware of one, but perhaps someone else is.
HTH,
/Uffe