How to include custom non-system folder under This PC in Windows 8.1?

Take a look at the tools FolderTweak for Win8.1

or This PC Tweaker

Both tools can add new items. Maybe they help you.

You can also add folders by right clicking and selecting "Add Network Location." You can use a network path like \\computername\users\me\whatever to point to a folder on your local machine.

Then

I recently figured out how to do this manually and have added the answer to this question: https://superuser.com/a/914701/422795 - "How can I create a new system folder to show up in “This PC” in Windows 8.1 without additional tools?".

It is not for the faint hearted though as it involves a lot of registry editing. However, if you're in a situation where you cannot download a 3rd party app (e.g. at work) and desperately want to add your own custom folder, it might be of help.