This can be done with action chains plugin, but it requires you to add
this custom action to the plugin first (copy/paste into the module editor).
After that import the chain from the zip file attached here (action chains > customize plugin > right click > import). After importing you should find a menu entry called "Add to series and update #latest_item". Run the chain from the menu entry after selecting the book you want to add to the series. A series dialog will pop up asking you for the series name and index, after adding them, the chain will update the #lastest_item column for all books in the series.
The chain has actions that use calibre template language, and it also sets conditions on some actions, so it might not be easy to understand if you have no experience in template language and action chains plugin.
Edit: Always use first on a test library. Never use it without backup until you are completely satisfied it works as expected.