Hi Shivam,
I would suggest this:
For the Excel importer use a validation flow where you check if the zone already exists (GetOrCreate flow). If the Zone exists, fill the association with the Office. Do this for every zone and office and then you’ll be able to use (for example) a Reference selector on a page where you only see the associated Offices per Zone.
With this setup you need to hve a line for every Office in you Excel file, otherwise it won’t work. Hope this helps you.
With kind regards,
Bryan de Wit