Log into Innovator with Admin rights
Under the TOC expand Administrator
Find the Revision Itemtype (if you don't see it you will have to add it to the TOC)
Search Revisions (Should bring up 'Default')
Open and Edit 'Default' Itemtype
In the 'Revisions' textbox append to the list of existing revision numbers that you wish to have.
Save Lock and Close
Should now be able to revision past 'Z'