You are right that there should be an API for users to be able to modify the tdf files of the dictionaries. In appreciation of your suggestions, I have awarded you some Telerik points.
As for a solution to the problem, you can submit a support ticket where we can give you a few source files along with an explanation how to use them in order to be able to add words to the tdf dictionaries. Removing them is trivial, let us know if you don't know how.
the Telerik team