You can make your own custom fields in AC10, but not in AC9/PM9.
There is nothing stopping you using the exsiting autotexts for a different purpose. I used to recommend filling out the information to serve as a prompt, as part of the CAD template. For example typing the Contractor name as 'CONTRACTOR' in, say, the Project Status field.
This can become quite intuitive once you get used to it, because you can just look down your Book Info for the appropriate prompt and change the it. It doesn't really matter that it is in the 'wrong' field. If you have run out of fields though, I'm afraid you're S.O.L.
Cheers,
Link.