Multiple characters in one portfolio

[Sturmrider]

I have created a new portfolio called Added encounters for monsters, and NPCs I create. The issue I am running into is once I put the first NPC in, and then open the file to put others in, it does not seem possible. I always have to create a new portfolio. It does not have a section to just add a character.

I know this can be done, but must be using the wrong search terms on the forum. Haven't had any luck. Any help will be appreciated. Thank you very much.