Hi Giuseppe
With this BAdI, you can change the planning elements when they are read by MRP from the database. This change will be considered during the MRP calculation, however, this change is not saved into the database again. That means, the reservation itself is not changed. Therefore, I don't think that this BAdI will be useful for you.
I wrote the following document with the most useful MRP BAdIs, however, there is no BAdI to change reservations:
http://scn.sap.com/community/erp/manufacturing-pp/blog/2013/08/12/badis-for-mrp
If your MRP is generating stock transfers, maybe MD_PURREQ_CHANGE can do the trick.
I believe that, instead of creating a custom code to change this standard logic, you can use a storage location MRP area. The storage location planned separately runs a very simple reorder point, which does not consider external requirements, therefore, the quantity is fixed. On a storage location MRP area, however, you can define the MRP type and the desired lot sizing procedure.
Check our on-line help:
MRP Area - Material Requirements Planning (PP-MRP) - SAP Library
BR
Caetano