Hi Javi
For reuse entities, you can either assign one entity or the reuse backend table (MARC, ...) to a UIBB. Assigning multiple entities, mixing flex and reuse, to one UIBB is not possible.
For extending the model with a flex entity, please check the guides in http://scn.sap.com/docs/DOC-7858#section2 - especially the UI guide http://scn.sap.com/docs/DOC-30192 . SMT mapping is not necessary. Hint: To check whether it's a UI issue or whether the definition of the flex entity itself has an issue, you could write a test report using the Convenience API and check the staging area.
Regards, Ingo