FI Subsitution OBBH for GL acccount or amounts via user exit

Hello,

For the FI document generated from a GR receipt from PO, I was wondering if it was possible, through the user exit "step" option (through subsitution process OBBH) to replace the GL account to a default account (defined in exit) OR change the price ( again, defined in exit)  based on the Purchase Order document type for the receipt?  I know those fields come "excluded" in GB01 but it can be maintained to be included.