Possibility 1)
If you import into a calendar which is connected to a syncadapter (i.e.: gmail,…), events become editable after it is synced by the syncadapter.
If you don’t like that just create a local calendar (“Edit calendars” -> “Create calendar”) and import into this one.

Possibility 2)
You’re imported events maybe unmodifyable because the imported organizer is not the calendar owner.
To fix this select the option “Discard organizer” in the “Import settings”.
If you import you’re events with this option you should be able to modify the events.

Posted in: Import