Hi Nadav,
Thanks for your interest in syncfusion products.
You an achieve your requirement of adding a combobox to the baritem by using a MainframeBarManager and add a comboboxbarItem instead of barItem or u can also add by dragging the control you want to add on to the MainframeManager.
sample: http://www.syncfusion.com/downloads/Support/DirectTrac/108181/sdidemo592118633.zip
Please have a look at the sample and please let us know whether we have understood your requirement and if not can you provide us a screenshot of the requirement needed.
Please let us know if you have any concerns.
Regards,
Paul Anderson S