If you really need to import rather than subscribe:

First create a new empty calendar. Then make sure to import whatever you are importing into that calendar. 

The import process may also give you an option to create a new calendar while importing. This works too. It doesn't matter which way you do it - as long as you keep each calendar's data in its own calendar.  

Here's a short video showing creating a new calendar on an iPad.  Watch the bottom left corner for the clicks:

https://www.youtube.com/watch?v=SR8ok9EZ5K0

Here's the Google instructions page:

https://support.google.com/calendar/answer/37095?hl=en

You can import to as many calendars as you like. You can edit the calendars because they are yours. If you make a mistakes, you can just re-import them into a new calendar.

Once you get a few different calendars set up you can watch this video on how to see them all at once (it's for Android but similar to iPhone):  

https://www.youtube.com/watch?v=Y_VqzxVqO44

More Questions Answered

For Google's instructions see:
Google's own Calendar Instructions