It is not difficult to solder to a threaded rod and, you do not have to use silver bearing solder, regular electrical solder works fine. The main thing is to have the threaded rod very clean. Use alcohol or even better, Acetone, to clean off any oil or grease on the rod and coupler first. Put a bit of paste flux if using electical solder (if using the silver solder use the flux supplied with the silver solder). Slip the coupler over the rod in the position to be soldered. It is best to make a jig to hold everything in position at this point. Wooden clamp type clothes pins work well here but it is quite easy to make your own jig with some scrap wood and clamps. Now, after all is in position, heat the rod and coupler where they connect until solder held on the junction melts and wicks into the joint. Remove heat and let cool a bit before removing from the jig. If you used the silver solder, wash the joint thouroughly to remove the acid else you will soon have a rusted rod. The flux from the electrical solder will not rust the rod but you may want to wash it away for appearances sake. Any connection made with electical solder will be adequately strong if done properly.