Is there a way to make up my already existing lessons into a collection?

Can’t figure out, if it’s possible to do.

Yes, you can do it.
Click on Lessons>Import, then on the title of the lesson. When the lesson opens in the edit mode, click on “Advanced settings” and then on “Choose collection”. You will be able to select an existing collection or to create a new one.
I hope it’s clear.

Thanks, Michele!