Create a surface that intersects the tooth at the edge of the PDL. Use that surface to split the faces of the tooth. Create a new Component. Copy and paste the faces on the tooth that touch the PDL. Use the Pull tool to create a solid body with the desired thickness of the PDL.
There are many other methods if this doesn't work such as making a copy of the tooth, cutting the top off with the intersecting surface to end up with a solid PDL body, then using the Scale tool to pull the solid body to a larger size. Then subtract the tooth from the PDL body.