Thank you for your interest in RadControls.
Actually the control you are looking for is the DropDown button. The difference between this one and the expander is that the expander expands its content inside itself (this is why the toolbar enlarges too) while the drop down button expands its content inside a pop-up. Unfortunately we still not released such a control. However this control is already in production phase and we hope it will be available for the Q3 release.
We hope this information will help you.
All the best,
the Telerik team