Pour out 12 ounces of tequila from the bottle; reserve for another use.
Add key lime juice, orange liqueur, and agave nectar directly into the tequila bottle. Replace cap and carefully shake until well combined.
Place the bottle in the freezer door. Freeze for at least 4 hours.
To prepare glasses, place graham cracker crumbs on a small plate. Dip rims of glasses into sweetened condensed milk, then dip into crumbs to coat. Fill glasses with ice.
Remove bottle from freezer and gently shake to recombine. For each drink, pour 3 fluid ounces of the margarita mixture over ice in a cocktail shaker. For a sweeter drink, If desired, add 1 to 2 teaspoons of sweetened condensed milk. Cover and shake for about 1 minute to chill and dilute. Serve over ice. Garnish with key lime slices. Recipe developed by Barrett Heald