We have discussed adding such functionality but since the designer can and probably will be customized, inserting code into the file is not a good option. In cases where you haven't customized the designer, you can just generate a new one.
I would recommend adding additional properties manually. You could go to our documentation about Creating simple control designers
. You can also use Thunder to help you add the additional properties by generating a new designer from which you can copy the logic for the new field(s). All you need to do is delete the designer attribute from the control, build the project, create a new designer somewhere, copy what you need from it and delete it. Then you can replace the parameter in the designer attribute to go back to using the old designer and build your project.
Please let me know if I can be of further help regarding this issue.
All the best,
the Telerik team
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