You must be able to setup and change the mapping (User interface)
What do you mean by domain to domain? If you just want to copy objects, you can simply create new entities and copy the values of the source objects to the new ones in a microflow.
From what I gather, you want to do the following: You have an XML-Domain mapping and a Domain-XML mapping defined in the modeler. You want to enable users to login with their browser and map certain entities to entities that are supported by the Domain-XML mapping, and vice versa for XML-Domain.
In order to accomplish this you need to somehow reflect the metamodel in your browser (I believe there's a mxmodelreflection thingy in the appstore) and then let people draw some type of associations between those. Then you need to actually apply this 'mapping' to the data at hand, probably via a java action.
Hope this helps you on your way.