Attach the vertices, than go to clean up option box and only click on nonmanifold geometry than hit apply. After that some faces might light up just hit enter, than try and covert to sub-d.
If that dont work it soulnds like you have duplicated faces that are giving you the trouble.
I am enough of an artist to draw freely upon my imagination, knowledge is limited, imagination encircles the world. (Albert Einstein)
https://www.artstation.com/kurtb