it is possible to change the default drop down list for categories with your own control. Here is what you will need to do:
1. Create a new control (user control or custom control, whichever you prefer). This new control will be used as the user interface for selecting categories on the admin side.
2. Make sure this control implement ITextControl interface. The Text property which ITextControl mandates will be used by Sitefinity to acquire the selected category when user clicks "Save"
3. Implement the control fully. I've attached a sample in which I've created a control which uses RadEditor for user interface of a meta field. You'll be using similar approach, except that you won't have RadEditor but rather checkboxes.
4. Modify the insert and update templates of the module you want to use this new control for selecting categories. For example, if it is News module, you will open ~/Sitefinity/Admin/ControlTemplates/News/ControlPanelInsert.ascx and ~/Sitefinity/Admin/ControlTemplates/News/ControlPanelUpdated.ascx. Locate the CategoryField control there and replace it with your new control. Make sure that your new control has the id="Category" (that is how Sitefinity knows for which meta field this control is used - id must match meta field key).
And that should be it. I hope you'll find this information helpful. Let us know if there is anything else we can do for you.
the Telerik team
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