I sort of see what you are trying to do but as far as I can tell there is no mechanism to do what you are asking. Originally posted in cdnusers.org by mcatramb91
You can make the tolerance field a USER Defined field, which would force you to specify the tolerance for ever hole size used on the design inside of the default-mil.dlt file, I would not recommend this but it was worth mentioning.
The first suggestion to use MANUFACTURE > NC > DRILL CUSTOMIZATION is the easiest and best way to go and you can change all the tolerances that are +0/-0 to your default tolerance. The other added benefit in using the Drill Customization is the tolerances defined in the database will be added to the NCdrill files generated out of Allegro so there is no confusion or conflicting information between the NCdrill data and the NCdrill Legend on your documentation.
I guess a good enhancement request would be "Any Padstack with a tolerance defined as +0/-0 should be replaced with a default tolerance specified in the Allegro .brd database" or "The ability to globally replace all the hole sizes with a tolerance of +0/-0 inside of NC Drill Customization with a default tolerance" - Currently the Drill Customization global change applies an updated tolerance across all hole sizes regardless of what they are defined in the library.
Hope this helps,