You cannot extend a PDSE member by opening the PDSE for output and positioning to that member. If you used POINT for positioning, the next write would result in an I/O error. If you used FIND for positioning, the FIND will fail with an error return code. To extend the member, rewrite it while open for output and issue a STOW REPLACE.
When you rewrite the member, you must provide two DCBs, one for input and one for output. Both DCB macros can refer to the same data set; that is, only one DD statement is required.
Because space is allocated when the data set is created, you do not need to request additional space. You do not need to compress the PDSE after rewriting a member because the system automatically reuses the member's space whenever a member is replaced or deleted.