Ok I'd say it would be a permission thing then.

Try running a test, create a second calendar on your own Outlook profile by going into your default Calendar, click the Open Calendar button located on the Home ribbon and select Create New Blank Calendar. Give the new calendar a name such as Test and make sure it will be created within your default Calendar. Click OK and it should appear as an option to view. Now repeat the Import process you tried before and see if it appears as an option to import to. If it does then it's an issue with the permission or setup of the custom calendar. If a specific user created the calendar then that user may need to perform this function from their account but it just depends how and who created the custom calendar.