Looks like @tomislavvv already linked you to my earlier forum article, but you’re right: the key is that you need a trigger for existing entries. Normally, it would be something like “publish Wordpress → import to Webflow” but since they’re already published, you need something else.
In my example linked, I pulled the whole dump of articles from Wordpress (or wherever) as a CSV into an Airtable database. There, I “triggered” each entry by checking a box, which was a custom column / field I made for each record. So basically just run down the list checking the boxes, and when Zapier checks every 15 minutes for changes, it notices that a bunch of them have changed (been checked), and triggers an import.
Works like a charm! I did have an issue where the import stalled because it was moving so many items in the zap, thinking it was an error. But zapier sent me an email alerting me and I went in and manually approved the transfer, and it was fine.