Hi Vlad
This not possible on the standard system. When you create a storage location MRP area, it means that this storage location will be planned independently.
Considering the stock from the storage location MRP area under the plant MRP area may lead to uncovered requirements on the plant MRP area. Nevertheless, if you want to do that, you can use method CHANGE_MDPSX_MARD of BAdI MD_CHANGE_MRP_DATA to increase the plant stock.
See an example of this BAdI on the following document:
Making an element not relevant to MRP or MD04 using BAdI MD_CHANGE_MRP_DATA
BR
Caetano