Create Subcategories for Directory Listings

Hi,

I have a huge list of categories because I have lots of subcategories. Is there a way to create main categories ans subcategories?

Example of Main Category: Surgery
Example of Subcategory: Plastic Surgery

I need to show only the main categories in that drop-down list. I'm using Advanced Post Search plugin.

Hope you guys can help me as always.

Thanks,
Jose