I can't add contacts from a running campaign to a new campaign, how can I do that?

The only way to add a contact from one campaign to another is to wait for the campaign to be finished (or stop the campaign and archive it), then you can check the box in the import contacts screen to force import the contact to the new campaign.

As a safety feature, the platform prevents you from adding duplicate contacts to multiple campaigns at the same time. Another workaround would be to create a separate client and run your second campaign from there.