A standard lens on an SLR camera is 50mm. A short telephoto lens is 135mm (used for protraits). A lens for landscapes etc. would be about 35mm.
This means that the zoom lens you have is just about capable of most of the photographs the average person would normally take. In the same terms as your other camera, it is a 3.75x zoom.
Converters - depends on how good they are and what they cost compared to your other option, which is a 100 - 300mm zoom lens. A 2 x converter will make your current lens into a 80-280mm lens. Unless you want to photograph using a tripod and stay a long way away from the subject, they will do. I'm sure you understand the difficulties of using your other camera at the extreme of its zoom capability without a tripod or other support.
In my case I went for the 100-300mm zoom, to go with my 28-110mm, in the belief that I would get better photographs