Don't confuse the teleconverter for dslr with the teleconverters used on a P&S.
The P&S types are add-ons to the lens that magnify or multiply the focal length. Those don't work well with a dslr or ILC. Even the Raynox brand might not be that good with a dslr.
The teleconverter you would use with a dslr is more like the extension tubes that fit between the body and lens. It's probably less expensive and more practical to just buy a 55-250 lens rather than add a TC to an 18-55 if that's what you had in mind.