Yes, you can use an outdoor grill in the same manner as described in the recipe for a stovetop grill pan using medium-high heat, which is about the equivalent of 375 - 450 degrees F. (Here is a nice overview on grilling temperatures if you need more guidance.) Don't forget to oil the grill grates very well so the halloumi does not stick, and monitor the ingredients well during cooking to make sure they don't burn, since outdoor grills can have hotter and cooler spots.