Is it possible to attach a heavy-duty magnetic tube clasp to a chain necklace? Could I use a headpin with a bead for the inside then make a loop with the remaining headpin to attach the chain on the outside?
Yes, this will work as long as the bead you use in place of the knot does not slip through the hole in the tube. The largest size magnetic tube clasp Fire Mountain Gems and Beads offers is 12x6mm, has a 1mm hole size and will accommodate a size 15 or size 11 seed bead or a 2x1-2x2mm crimp bead.
Tip: Choose precious metal beads and findings, like sterling silver, to prevent your magnet clasp from being attracted to the beads and findings while you work.