there is NO import option for local users on FortiGate (from any format, not just from CSV).
Such option exist only on FortiAuthenticator, but that's different product and 'league'.
However as mentioned here ..
first, via direct integration you will save a lot of headache as your users will have separate passwords on O365 and on FortiGate. Plus you will have to manually set all those up and maintain their group membership. So maybe integration is not that bad idea and is quite used solution, as it let's you drive all the permissions from AD.
CLI config of 'config user local' is pretty simple and even simple bash/MS-cmd script might be enough to generate that config section, then copy and paste that to config backup from your unit, and restore such enhanced config back.
Groups can be handled as well. That's simplest way.
As you mentioned O365, then maybe you do not have Domain Services in Azure, to make LDAP integration. But even without that you might consider SAML integration. But it's again a bit more complicated then plain users in local storage on FortiGate, but similarly flexible to point 1.
If you want to learn more about that then check https://docs.fortinet.com
and FortiGate integration with SAML.
4. not mentioned before but I can't keep that out .. how about to upgrade 6.2.4 (released year ago) to something more recent, or do you have some serious reason for keeping that old version ?