When building a Series, you can select Copy existing messages in series from the More drop-down menu:

Then, select the message you want to copy from the list:

Your message will then be copied, with its filters added as a Rules block:

Hello

How do I make this feature appear in the More menu on my series? Right now it's not there:

thank you

Hey @virginie b​ 👋

It looks like you are not currently in Edit mode, you'd need to select Edit mode and then the option to Copy existing message into Series option will appear 🙌