Do you have more information about what is failing? I've just tried a project like yours here, and the combobox is correctly preserved:
FlexCel fully supports them, and will even convert them from xls to xlsx to xls. You can even change the selected item from FlexCel.
What we don't preserve is ActiveX controls (those are below the "forms" objects in the Developer tab of the ribbon. But ActiveX controls have been deprecated for a long time already, they look and behave like win3.1 controls, and there is no really a valid reason to use them instead of the form controls.
But from what I see in your screen shot, it looks like a "Forms" dropdown, not an activex one. If this is the case, I am not sure on why it wouldn't be preserved. If you want you can send me the file to email@example.com
and I can take a look.