As I understand it, you want a "plain 1.0X teleconverter" (which would have to be called something else because it no longer adds to the tele nature of a lens, nor does it convert anything) merely to "seal" and protect the sensor...
No such thing exists for a modern DSLR. Nor should it because it would be a bad idea for two reasons:
1. Adding optics in that manner would reduce the image quality potential of every lens you use. You simply cannot add more glass without some optical "cost".
2. The sensor is already sealed and protected. Every DSLR made has some form of filter in front of the sensor. When you "clean the sensor", you aren't actually cleaning the sensor itself. You are cleaning a filter that sits in front of it. Most DSLR sensors have an anti-alias filter, designed to slightly blur the image to reduce chance of an optical effect called "moiré". All have some sort of coatings on the filter, too... to reduce reflections and, in many cases now, to resist dust adhering to the filter.
When you mention magnification, some responders are immediately thinking of macro extension tubes, which have no optics inside and are merely a spacer that moves the lens farther from the sensor plane so that it will focus closer (i.e., give higher magnification), but at the cost of no longer being able to focus to infinity while the macro extension tube is in place.
I would simply suggest you learn to clean your camera's sensor (okay, okay... actually you'll be cleaning the filter in front of the sensor). It needs to be done carefully, but it's not rocket science. There is comprehensive info about cleaning sensors available at
http://www.cleaningdigitalcameras.com/ Alternatively, have the sensor professionally cleaned periodically. It's not very expensive to have done and most of the time other camera service and cleaning is performed at the same time.
If it really, really bugs you... buy a fixed lens camera. Get a point-n-shoot with a non-interchangeable zoom lens. If the range of the zoom lens isn't wide enough for your purposes or it doesn't focus close enough for macro shots, you can get auxiliary lenses that attach to the front of the camera's lens to make it's angle of view wider or more telephoto or closer focusing.
Incidentally, when using a "real" teleconvter (which can be found in 1.4X, 1.7X, 2X and 3X strengths most commonly), the lens' closest focus distance doesn't change at all, but it's magnification does change by the TC's multiplier. For example, if a lens focuses to 18" and produces 0.21X magnification at that distance (about 1/5 life size), if you add a 1.4X teleconvter to it will still focus to 18", but now will render about 0.294X magnification (a little less than 1/3 life size).
Conversely, a macro extension tube without any optics does not change the lens' focal length, but does change it's closest focusing ability, and the degree of magnification along with it.
The idea of some sort of "lens base" with interchangeable "lens heads" isn't a new one, by any means. In my camera collection I have a lovely Zeiss Ikon Contaflex Super B from 1963 that's designed exactly this way. Instead of the entire lens being removed and replaced to change focal lengths, part of the lens assembly remains attached to the camera all the time and five different interchangeable "lens heads" are available to provide different focal lengths (35mm, 50mm, 85mm and 135mm) and 1:1 macro. (
http://camarasclassicas.blogspot.com/2011/02/contaflex-super-bc.html)
In the late 1950s and early 1960s there were a number of camera manufacturers offering SLRs similar to this, mostly done to keep costs more reasonable. The problem is that it seriously limits the range of focal lengths that can be offered. One advantage of this type of design was that it allowed for an in-lens leaf shutter (instead of the focal plane curtain or blade type shutter most SLRs use). Unlike other types of shutters, leaf shutters allow flash sync at all shutter speeds. But they have limited top speed. Most are 1/500 tops... a very few get to 1/1000 (compare to modern focal plane blade-type shutters that can do 1/4000, 1/8000 and in a few cases even faster speeds).
Leica even went so far as to offer a series of APO-Telyt R super telephoto focal length "lens heads" for use on different "focus modules", to provide 280, 400, 560 and 800mm lenses. (
http://www.apotelyt.com/photo-lens/leica-apo-telyt-r-module) But these didn't do what you want either... The focus modules mounted to the camera via bayonet, to be able to swap out and use other R-mount lenses on the camera.