No this is not possible because for each diagram you get a model or you
can use GMF model for project level then try to create xmi from it. The
problem is that this GMF top XMI mapping doesn't work !! It means that you
get nothing really usable at XMI level !!
I also really don't see where is the metamodel with Topcased, you
certainly mean either model at diagram level or GMF model but certainly
not metamodel.
Would you like I give more explanation on this model, metamodel
tranformation stages it seems that there is some confusion here ?