I have to ask here since the editor's help links to a dead wiki.
I have figured out quite a bit by myself yet, but there are some things I couldn't get yet.
-How do I create or edit equipment/ spells? Do I really need to unpack and use an external editor?
-In the dialog editor, how can I link to an existing line?
-I had problems with a new variable I created for an existing content pack. Couldn't set a value in the dialog editor, the window just didn't open.

Can't help you with your questions, but the wiki is here.

You do need to use an external editor to create or modify items (including spells). There isn't any way to change them within the editor. Starting with Dragonfall, you can use the "Save Project as Source" option to export all of Dragonfall as text files, which you can then edit with any text editor. It's easy to change descriptions and values, harder to change how items work.

You link to an existing line by copying the destination node, then choosing "paste as link". The wiki has a decent overview of how to do this. This old video uses the original editor, but the process is still the same.

You might need to make a copy of the existing content pack. You can't really make changes to the built-in version of Dragonfall: instead, you will make your own copy of Dragonfall, make changes to that, and then launch your new version.

Thanks for the info and the link.
Yep, I've figured out that you can save as source and then copy or edit the item text files, but I thought there might be a better way.