Unfortunately, currently there isn't a way to tell SQL Prompt to use tabs instead of spaces for laying out the code. However, if you do want to convert them after laying out the code you can try the following:
- Highlight the section of code
- In Management Studio go to 'Edit' > 'Advanced' > 'Tabify selected lines'
This should change the tabs from spaces to actual tabs. The developers are currently working on SQL Prompt 4 at the moment and as I understand it there will be an option to keep tabs in the formatting options. SQL Prompt beta will be released Q2 2009.