Do you mean the Tamron 18-250? I have always found that these cheaper all encompasing lenses are built down to a price and won't last a lifetime, although it gets a tolerable review here
All Tests / Reviews
But don't even think about using a converter if you want good quality results especially as the sharpness tests in that review are only average at the full zoom end.
Once again, I'm afraid that there is no cheap alternative to good quality long lenses; except secondhand, and then you need to be sure that you are getting a good item which hasn't been dropped or subjected to some other traumatic experience.