Numerical Innovations - Help Center

The command Convert To Drill Layer... (menu:  Fabrication | Generate Drill/Rout | Convert To Drill Layer...) would be perfect for creating a new NC Drill layer with slots, etc.  --- assuming you have an existing Gerber file that already contains everything you require.

Here's a quick tutorial,  

If you don't have anything, please place FAB 3000 into NC Mode, and you will have a wide feature set to add objects like drills, slots, etc.  Here's a quick tutorial showing me create various NC Tools and randomly add both drills and slots,