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