How can I manually add a contact into a segment?

We run a campaign in Customer.io that is triggered when a customer signs up for our app.

We have a new type of user that we want to run a different campaign with, however.

As of now, we need to manually identify when this new type of user signs up.

My thinking is we can create a segment in customer.io for this new type of user so that we can exclude them from our main campaign. And add them to a new campaign.

Is that the best way to achieve this?

If so, how can I manually add an existing contact/user to a segment (so that they are excluded from campaign and added to another)?

Thanks!