There might be a way to do it, but tbh it sounds messy. You'd have to fully encase the sandwich or you'll have grounds falling out into the coffee. So you'll need a lip on the bottom filter to act as a vertical wall. Then you'll need a rubber gasket to get a tight seal on the bottom so grounds don't go around the edges and later get in your mug when you pour. The whole apparatus will undoubtedly be dripping as you take it out, so it needs to be done over the sink or trashcan.

At that point, might as well get a thermos.