Hello Lucien,
I am not sure if I understand you correctly.
Generally, it is possible to set different data contexts to separate parts of RadRichTextBoxRibbonUI. In case you are trying to set a DataContext to the dropdown in your application, but still execute one of the RichTextBoxCommands, you can bind the command using the element name of the RadRichTextBox in your project:
I hope this helps! If I misunderstood your requirements please get back to us with more specific details and we will do our best to assist you.
Regards,
Petya
Telerik