If the PDS exists, then use the following syntax.
"alloc da('YOUR.PDS.NAME(MEMBER)') f(file03) mod reuse"
If not refer the example 7 of the following link.
Refer the link for allocate command syntax. This has a lot of examples for your reference.http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/BOOKS/IKJ4C531/1.7?DT=20021022140245
Post if you face problems.