Immitation Bellows

Yes, the lens is doing the same thing optically as an extension tube would. It'll be limited because you're handholding it, but it's a pretty good illustration of the fact that "acceptable" solutions don't have to cost a bunch of money.

This arrangement is called a "stacked lens". A similar arrangement is used in many microscopes, with a fixed-length telephoto (usually 200mm) and the microscope objective acting as stacked lenses. There are even a few microscopes (the Bausch & Lomb MonoZoom-7 comes to mind...) that use a zoom lens with microscope objective to allow variable magnification. This is what you have created with your arrangement, Gene.

The term used for the telephoto is "tube lens". The usual method is to set the tube lens to focus at infinity and to open its aperture all the way up so it does not vignette the image.

Magnification you can achieve is a simple calculation...

M = FL(tube lens) / FL(objective)

I'll assume you're using a 75mm enlarging lens. Then your 60-300 zoom will give you a magnification range of 0.8x to 4x.

A nice thing about stacked lenses is the objective sets the general quality of the image. Use a good enlarging lens as the objective, and pretty much any zoom or fixed telephoto will give you a good image.

Another nice thing is these arrangements can be parfocal versus magnification. The working distance is set by the FL of the objective, not the overall magnification. The tube lens just acts as a variable multiplier of the image created by the objective. The system is completely parfocal as long as you focus the tube lens at infinity.

Google "stacked lens" or "infinity corrected lens" for more info on this