I depends how the combobox is added to the state. If it is
created in that state, you can't, since it may not yet have been
created (unless you've already been in that state). Your best
option (don't know why you're trying to do this, though), is to add
the control, hidden, in your base state, and then show it in the
state in which it should be visible. That way it's created right at
the start and you can use it.
You should view states as a 'script' of changes that are made
to the base state. These changes include adding/removing children
and modifying properties. When adding, the children will be created
and added (or reattached if they were previously detached). When
removing, the children are detached from their parent.