Even though the manufacturer doesn't offer a trim kit for the specific microwave you want doesn't mean that you can't build it in with a trim kit.
There is no functional difference between a "countertop" or "built-in" microwave, they are the same type of microwave, and TrimKits USA will either have a "standard" trim kit that will fit perfectly with your cabinets, or can build you a "custom" made trim kit.