Can I create a custom taxonomy / post prefix?

I've searched the forums and haven't found a good way to add a custom taxonomy or additional field that acts as a forum thread prefix. For example, I don't want to have a main category and subcategory. I'd like to have a main category of "Ford Cars" and have all my posts on that car in the category. However, I'd like to require every user to tag their post with a dropdown list of entries for each category - it could be a tag or custom taxonomy. In this example, the user would need to choose from a list of "Taurus, Thunderbird, Mustang, etc." This way I could search categories also by this additional tag or added field. I would use Gravity Forms to create a new post type and insert that dropdown as a required field. Was wondering if someone could help.