As you can see from the PITS issue
, so far there is no property that could indicate whether to open the DropDown up or down. Nevertheless there is a workaround that you can try as you can create a custom RadComboBox and overwrite the OnApplyTemplate method in order instruct the Popup to show on the top instead of as normal. Also some more adjustments need to be done on the animation so it would not start to close in a wired way from bottom to top.
The described workaround is shown in the attached project it could be great if you can check it out and let us know if this works for you.
All the best,
the Telerik team