Hey Barnabas,
Thanks for reaching out to us!
Sure! This is definitely possible. The current iCal feed will essentially pull the month view so it will always be the current month. However, you can add the following snippet to your theme’s functions.php file (or a plugin to make it easily accessible by all sites within your multisite network) https://theeventscalendar.com/support/forums/topic/ical-export-entire-calendar/#post-1088880.
Then, for your iCal Import onto the main site, the feed would pull from “/?ical=1&year-feed” URL to get the full 12 months.
Let me know if this helps.
Thanks!