You need to pick to a folder for Obsidian to store notes in that iA Writer can also see. Then just move all the notes from the iA Writer default folder to that shared folder.
I use iA Writer on Mac and iPad, so I had an iA Writer folder in iCloud. I made a new top level folder on iCloud for my Obsidian vault, and then added that folder as a location in iA Writer.
There are a bunch of different permutations possible here, but really all you need to do is get the files into a place that both apps can see.