Hmm. I am sorry, I only can tell you that I have this Cross-Origin resource sharing (CORS) option (I have to scroll down in Application Core and it is next to Miscellaneous). And also that the setting was essential for me to be able to migrate Pydio 8.2.5 to Cells together with the Apache settings. Why is still a mystery to me. (Then also make sure that you run the Pydio Cells Data Import via the given URL).
But we can try find it together: What Pydio version are you running?
Are you sure the migration plugin is successfully installed? (Check it in Pydio 8 → Settings → All Plugins → Available Plugins → Action Plugins → Migration. Just noticed and whoever cares: the link to the documentation of the Migration Plugin is broken). Make sure the plugin exists and is enabled.
I am not aware of an “Advanced Settings” option in Pydio 8 (Pydio Cells does have a “show/hide advanced parameters” toggle switch), but I will keep looking.
I wish you luck! I am still struggling with the migration ([1], [2]). It’s much harder than expected. 
Please note: I wanted to edit my post and wanted to remove PUT from Access-Control-Allow-Methods from the Apache config (because only GET, POST, OPTIONS were given in the Pydio 8 / CORS settings page) as my migration worked without PUT. Don’t know if it is harmful, though. So this is how it should look like:
Header set Access-Control-Allow-Origin "https://pydio-cells.intern.example.com:8080"
Header set Access-Control-Allow-Methods "GET, POST, OPTIONS"
Header set Access-Control-Allow-Headers "Authorization, Content-Type, X-Requested-With"