YR,
???? Product availability ignores dynamic pegging. ATP has its own time series, which are not directly related to dynamic pegging areas.
I suppose the most likely solution would be to enable fixed pegging.
Once fixed pegging is configured, you can run a heuristic SAP_PP_019 which will create fixed pegging relationships between supply and demand. Within this heuristic it is possible to customize ("User defined settings") how the relationships are created. I have never created one which tied sublocations together, but I would think it would be feasible. I don't remember if sublocation is in the standard field catalogue, you probably would need the help of an ABAPer to get the sort fine-tuned to your requirements.
Once you have established fixed pegging, ATP will generally follow the fixed pegging relationships.
Best regards,
DB49