You can lock tables to keep them from being edited or moved. You cannot lock/unlock individual cells in a table. So, you can put in one table all the things you want to protect and put in another table all the cells that require data entry. That kind of solves the problem but the user can still delete, move, or otherwise mess with the unprotected table in ways you wouldn't want them to.
The iPad version of Numbers also has a feature called Forms. You create a data table (which you can lock) and a form for the data entry. This may be closer to what you are looking for.
http://macmost.com/creating-forms-in-ipad-numbers.html
I don't know how you plan on adding a signature block to your document