the tubes are bout 1cm ID with only about a 1mm thickness. I don't think threading them will be possible, and the seal between that has to be water tight.
you can get the coupler and glue from the shop where you bought the pipe. simply clean the edges of the pipe and apply some glue around the pipe and push it into the coupler
you need coupling or socket to connect the two pieces and then apply adhesive to connect, there is a traditional way which you heat up on side to form a spigot or cone shape little wider than 1cm and insert one end while it is hot then press for tightness, this is not good from quality point of view.
I would go for a coupler on the outside of the tube, which should be stronger and won't restrict the flow. Get some 12mm ID tube from a model shop or plastic supplier. Roughen the inside of the coupler and the outside of the ends of the tube with sandpaper, apply plenty of epoxy, assemble and leave to set. You might want to improvise some kind of jig to hold the 2 tubes in-line while it sets.
It really depends on the material you have, i would go and find a smaller pipe that fits in there tightly and then glue it in or you can find a sleeve that fits on the outside and glue it !
