At the bottom of the Inoreadear integration page, there's a link to an IFTTT applet for syncing Inoreader starred articles, which doesn't seem to work now. Assuming it was removed since searching didn't help.

I have published a python script to help with importing Inoreader's starred articles into Raindrop here: https://github.com/gursuj/inoreader-to-raindrop. For future articles, the send to Raindrop option in Inoreader can be used.