Intro: How to Re-insert a Thread Into Pants & Trousers
1) As you see, I simply bought me a flexible metal rod (for fences/gardening) and cutted a short portion of the insolation away.
2) So you can fold a small amount of thread over it and fix it with tape.
3) Then it's pretty easy to re-insert the thread due to the smooth insulation of the rod (green).
Hope this helps!