Add Subcategory dropdown to "Add Listing" form?

We have hundreds of subcategories in our directory, so selecting these from the Category dropdown while adding a listing can be pretty complicated. So instead, we’re wanting to create a second dropdown for “Subcategories” which will be enabled once a user clicks that category on the main dropdown.

**We have experience programming and do not mind getting our hands dirty – so even if we could just be pointed in the right direction, that would be great.**

Thanks in advance!