I don't know of any reason why it wouldn't be possible - you could setup your custom action server to create an action client instance in its callback function, and use that action client to send a goal to the nav2 server. You could presumably also pass the feedback from the nav2 /navigatetopose action server back to your own client so that it doesn't get lost in the process. Read through the ros2 actions tutorial carefully, and feel free to update this question if you run into issues on the implementation.