Ok, I found it. It was so simple that I didn't think of it in the first
place.
Just let the MweReader know, where your XMI-File is ;)
<component class="org.eclipse.xtext.MweReader"
uri="classpath:/model/myOwnModel.xmi">
<!-- this class will be generated by the xtext generator -->
<register class="org.xtext.example.MyAgentStandaloneSetup"/>
</component>