Hello Dominik,
Thank you for writing.
You can achieve this behavior by changing the editor type and using a
RadDropDownListEditor. The following section in our documentation provides an overview of the different editors also shows how they can be customized:
RadGridView | Editors.
I am also sending you attached a sample project demonstrating the suggested approach as well as a short video showing the result on my end.
I hope this helps. Should you have further questions please do not hesitate to write back.
Regards,
Hristo
Telerik by Progress