How can I put a choke tube in an unthreaded barrel of a Remington 1100?

It will be cheaper to buy a Rem-Choke barrel than to have your existing barrel threaded.

http://www.midwayusa.com/find?&sortby=1&itemsperpa...

No. If you want to use changeable choke tubes you will need to take your gun to a gunsmith and have the work done by a professional. The existing choke will have to be removed by shortening the barrel a little bit, then the barrel will have to have a new bead installed and threads cut for the tubes you intend to use; since not all brands use the same thread pitch you need to let the gunsmith know what brand you intend to use. Don't forget to pattern the gun with each new choke tube, as well.

The costs for this will vary by location; my local gunsmith charges $50.00 for an established customer, more for a first time customer because each time a new gun is brought in it gets a very thorough checkup and cleaning.

Shoot straight and stay safe.

Yes, I had mine threaded for $60.

Have your barrel threaded.

Duct tape it on