I'm working on a project that will allow a stockroom manager to mark locations as "allowed" for a given task. My Form's current state looks like this:

If the user checks boxes manually, or uses any of the 3 buttons at the bottom the Form works perfectly. The "Main Location" drop list acts as a filter to only work with a specific Location, and causes the Primary Collection (and Grid) to display all the possible Locations that can be selected.
What I need is some way for the user to enter/select a "start" and "end" location that are in the current Main Location. Ex: 10 00 01 through 10 00 358. Then have the three buttons perform their action on the range specified.
I was trying to allow the user to use just a Combo Box to set the values, but I'm hitting the record cap for entries. I'm wondering if I should just make it two Edit fields, tie it to Locations & then just run the check to see if what they typed in is a valid Location.
Is that the best way to process a range of values in a Grid? I was originally trying to do something similar to Excel with row selection but it is not possible.