Yes it is possible. You just need to change the playlist in the uTool insert the new song then overwrite or create a new playlist in uTool. Then you need to export the playlist again. The uTool will ask you to overwrite the existing songs or not.
When you select the “keep all” the existing songs not transferred again to the USB storage device, but the playlist and the new song will be copied. You can check on the “export completed” message which files (playlist and wav) transferred to the USB storage device.