You can’t add playlists (.m3u or otherwise) to the GoList, which is essentially an internal playlist itself.
And unless I’m mising something, I can’t see why you would want to. 
Just play the playlist. It will play all the songs in the list, regardless of folder location. Note however that .m3u playlists will only work from one memory location or the other (internal or external memory card); you can’t have tracks/files from both locations in the same playlist. For that you need to convert them to .pla format.
