You can find instructions on how to import ReqIF file to Capella in the user guide. I think it explains quite well: Home · eclipse-capella/capella-requirements-vp Wiki · GitHub
It may be helpful to read this additional forum post: Import/Export ReqIF - Capella - Eclipse Capella Forum (mbse-capella.org)
In addition, you may also wish to read [Capturing textual requirements in Capella complete guide](Capturing textual requirements in Capella complete guide )
I hope that helps,
Hélder Castro