Can "extensions" be put on a Canonet QL17?

This is a coupled rangefinder camera, so the lens attached to it will be rather integrated into the camera mechanics - no chance to hack that camera into anything that can usefully work with fully interchangeable lenses. Wide angle converters or teleconverters that can be adapted to the filter thread would potentially work, as long as they do not shift focus - also, it seems there is a metering cell besides the lens, but within the area a filter or attachment will cover, so it is likely that you would be restricted to fully manual unmetered exposure only.

This camera is simply not designed for using interchangeable lenses.

Consider leaving and using that camera as it is, and getting a second system that is optimized for using interchangeable lenses in a commonly available mount. Most rangefinders with interchangeable lenses are uncommon (Braun, Voigtlander) and/or commanding premium prices (Leitz) - probably the most common affordable ones are russian designs derived from Leitz. These are an acquired taste, and still not very versatile as an interchangeable lens system, compared to an SLR.

So your best bet would be a manual focus SLR system, based on eg a Minolta SRT-anything or X-500/X-700/XD-anything (these all take MD mount lenses, which are common in the used market), or Canon A-1/AT-1/AE-1/AL-1/others (takes FD mount lenses, also common), or an equivalent Nikon model (F-mount lenses), or something M42 screw thread based (various brands, most rather plain, also some upmarket models existed by eg Pentax or Fuji). SLR lenses are hard to adapt cross-system, going for lens mounts outside these four common ones could make it hard to find a good lens selection. Mind that you can easily adapt SLR lenses to your mirrorless digital camera (DSLM) too (but not really to a different mount SLR!!), so they can serve two purposes.