Way of importing zone file

We have configured some wideips on the GTM, e.g. www1.abc.com. We have another DNS server which is hosting other records of the same zone abc.com. We are trying to import the zone file from this DNS server to GTM. However, the only way to import zone file seems only when creating a new zone, but abc.com is already on GTM. How should we import the zone file to GTM?