Buffers will default to the contents of the current DataUnit, which means a single Cube/Scenario/Entity/Parent/Consolidation/Time combination. If you want all the records of the DU, just create a buffer pointing to A#All and you're done (although I would suggest RemoveNoData(A#All) at a minimum...). If you plan to filter that buffer later, look at the extended options of GetDataBufferUsingFormula in Intellisense, because IIRC there is a parameter to optimize that situation.
This said, it doesn't sound like a good idea. You will potentially retrieve hundreds of thousands of intersections in one go, using a lot of memory and likely including a lot of stuff that you will never need. I'd suggest to be more precise if you can.