Hi Ian,
Welcome to the Microsoft Community forums where you can find answers to all your Windows related queries.
We can’t import contacts directly to the People app from a CSV file. However, if you visit http://people.live.com/ you can import the contacts through manage option. You can refer these steps:
a) Visit http://people.live.com/ and login with the same Microsoft ID that you have synced with the People app
b) Click on “Manage” on the top and click on “Add people”
c) Click on “import from a file”
d) Browse and import the CSV file from your computer
Note: It might take some time for the contacts to be updated in the People app in your computer.
Hope this helps. If you have any other queries related to Windows, please get back to us and we’d be happy to help.