This is absolutely possible and dangerous for the engine. I would drain the system and
refill it (or have it refilled) using a vacuum as described, or alternatively choose the "manual" method for refilling. I cannot upload the PDF, feel free to drop me a PM for the procedure.
Check
https://www.pelicanparts.com/techarticles/Boxster_Tech/29-WATER-Flush/29-WATER-Flush.htm as well