The Standalone Designer tool is an application we provide in addition to Telerik Reporting libraries. The application cannot be customized, but it can be distributed as part of your solution - License Agreement
(check 2. Software License
Feel free to add a feature request
for having customizable Standalone Report Designer, where entries are considered based on the demand for them.
Let us know if you have any further questions.
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Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items