I have worked this out, and I'll explain the solution here in case anyone else needs it.
You'll need a repeating group in your form, which you can then use as the data source for the dropdown. (So this will be my:dropdownGroupRepeating with my:dropdownField within it)
You'll need a field for my:manager2 and my:manager3, and you'll be setting those values via a SharePoint web service.
When the value for my:manager2 is retrieved, you simply set the my:dropdownField equal to my:manager2. (no qRules involved here)
However, when you retrieve my:manager3, that's when qRules comes into play. You'll need to first Insert then Set Value:
Insert /parent=/my:myFields/my:DropdownOptions /child=my:DropdownOptionsRepeating /posn=last /count=1
concat("SetValue /xpath=/my:myFields/my:DropdownOptions/my:DropdownOptionsRepeating[2]/my:DropdownOptionField /value=", Option2)