You can model slabs of different thickness in STAAD.pro and still export the model for final design to Ram Concept, but if it were my model I would probably just use a single thickness simple slab in STAAD which is probably good-enough for lateral analysis and column design and then export the levels to Ram Concept where you can add the drop caps, tendons, design strips and punching checks.
Now, I personally think that the integration of RAM Structural System with Concept is a little tighter than the link between STAAD and Ram Concept, so if you already have the model in RAM SS you could continue with that. Since the program does not generate Wind or Seismic loads per the Indian codes yet you would have to put in use defined lateral loads. The biggest limitation I see is in the concrete column design within RAM SS which has a lot of concrete design codes, but not the Indian Standard.
