You need to remove the first lens to use a second one. If you got the 55-250, it will pick from where the 18-55 stops. So if you want to keep wide angle, you either need both, or as you say, get something like the 18-200. Note there is some risk to changing lenses outdoors, especially if you're in a dusty area as it will find its way into the camera, and eventually onto the sensor and impact the images.
If you look at 3rd party lenses, Sigma do an 18-250 and Tamron do a 18-270 for example, so they may be alternative choices. Just be aware that lenses with more zoom range in one go tend to be designed with more compromises, thus may have lower image quality compared to using separate lenses.
It may sound odd, but if you just want a lot of zoom range, a bridge camera like the Canon SX50 might do better. Of course there will be different compromises, but it has as much zoom range as you can imagine while still keeping a reasonable quality level. Worth a look at reviews of that.
_________________Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.