You can place an SWT control on the toolbar, but it doesn't work for menus. Here is the toolbar way:
Open the extensions tab in plugin.xml file
Add a menuContribution extension to org.eclipse.ui.menus and set it as a toolbar (toolbar:ID)
Add a toolbar to this menuContribution
Add a control to this toolbar
Double click and create the class which extends WorkbenchWindowControlContribution and fill the createControl(...) function using the desired SWT controls.
After setting the menuContribution as a menu, it is still possible to add a control, but unfortunately, it does not show any controls.