Yes and no. As long as there are no posts published yet in the ongoing project, you're still able to change channels. As soon as you post something in the project you won't be able to add or remove channels from the project anymore.
To add a new channel click the 'Edit' button at the top of your project dashboard or in the project popup menu.
Next, the edit screen for projects will open. Remove or add the channels you want to change and save the changes.
In case you did publish something already, we advise you to end the ongoing project and start a new one with all the channels you need.