At present, you can’t use the MySQL Workbench migration tool to migrate MySQL 5.0 to MySQL 5.5, as documented in Bug 66861. The documentation of the mysql.proc table is also different from the physical definition in MySQL 5.0 release. I logged documentation Bug 66886 to get it fixed.
The actual definition of the mysql.proc table for MySQL 5.0.91 holds 16 columns not 20 columns, and is summarized below: