Gianfranco, where do you see a drop column syntax for DB2 z/OS?
This the IBM technical reference for version 10
http://www.ibm.com/support/knowledgecenter/SSEPEK_10.0.0/sqlref/src/tpc/db2z_sql_altertable.html
Scripts created by Toad have been over %95 correct in my experience. What you are complaining about is simply not possible.
Yahya