Nicolas,
You cannot easily export a CMA and then import i into a SmartCenter.
In R77.30 you can export and import network objects, hosts, networks, groups etc. You can import those into another R77.x CMA/SMS, so this could be a path.
With Smartmove you can export and import policies and objects. This could be another path, use CPuse to upgrade the current Multi Domain server to an R80.x version and run the export from there and then build a new R80.x SMS to run the imports on.
I have also heard Check Point is working hard on getting the import and export utilities to move from R80.x SMS or CMA to any other possible flavor (CMA/SMS), if merge is also part of this toolset I do not know, as that is what you would need here.
Regards, Maarten