Hello and welcome.
The current iMac models don't have user-upgradeable storage. If you want internal storage larger than the 512 GB, you need to have it upgraded at the time of purchase.
I recommend that you simply invest in an external Thunderbolt drive. You can get pretty much any storage capacity you want, and it can be used in the future with other Macs, or if you replace your current iMac. Another good investment would be a second external drive to use for Time Machine or other backups of the data on our internal and external drives.
Hope that helps,
C