I bought my Raboesch 108-30 bow thruster with extension tubes. One pair of extension tubes was enough for me, but you can add more if you need to do so.
The extension tubes are "blind" at one end, but this doesn't matter 'cos you've got to cut it to length anyway. If you add a second extension tube you may need to router (or simply grind) a recess to one or other tube to get them to fit - a Dremel will do this job easily enough.
To me, the biggest advantage of using the proprietary extension tubes is in maintaining the same inner diameter through the full length of the thruster tube. Also, the extension tubes are the same material as the thruster itself, so gluing is not an issue. But you may want to add a layer of epoxy over the outer surface of the joint to ensure a waterproof joint - one of my joints leaked and it was a *female dog* to fix inside the hull.
Des.