Can I add mentions manually?

Yes, it's possible to add individual mentions to a particular project manually. In order to do this, you need to enter your project settings and then go to Sources.

In there, you should be able to see a button called "Add mention form":

Once you click on it, you will see a field where you have to enter a URL address of a mention you wish to add:

After entering the URL address and clicking "Next", you will see this:

The only thing you need to do is fill in the form and click Save to add a mention to the project. After that, they should appear in your results in your project dashboard.