Adding Episode to Series, or rather assigning Series to an Episode, is easy and can be done from either the Project's Episode overview or from one of your Series' Episode overview. Both work the same way.

Using the 'Series' column in the Episode overview, assign an Episode from the list to a certain Series, or create a new Series from scratch.

To move an Episode between Series, you will reassign an Episode to a Series. To do so, you can use the same 'Series' column in the Episode overview, then reassign an Episode from the list to a different Series, or create a new Series from scratch.