We hope to be able to bring the documentation for RadExpressionEditor online this week. Please excuse us for the delay and any inconveniences it might have caused you.
In its current version, RadExpressionEditor does not offer extensibility features like the ones you have described. We are aiming to to offer this and other features in a subsequent version, most probably our next major release in Q2, but I am unable to commit to a specific date. There is a PITS item for the feature you describe, which you will be able to find here
. You may use it to track the progress of the implementation of the feature.
the Telerik team