Unfortunately, this is impossible. The Flange Back distance on the RF lenses is 20 mm while that distance on EF lenses is 44 mm. This is the physical distance from the back of lens (usually a metal ring on L glass) to the sensor. With the RF being 24 mm closer than EF, you can use a 24 mm adaptor to adapt EF to RF mount. But, unfortunately, it cannot work the other way around because the RF lens would be moved 24 mm further away from the sensor, thus, a 50 mm RF lens would be 74 mms away from the sensor and the lens elements would not be properly focusing the image on the sensor.
Here is an article talking about this and other aspects of RF vs EF, Flange Back differences is illustrated about 3/4 of a page down.
Enjoy your "new" EF 70-200 III.
For what it is worth, I have yet to purchase a single RF lens. Still using the EF glass. Not surprisingly, it works extremely well on the R5. About the only flaw with this that I have found so far is that with video you can hear the USM motors while focusing the EF lenses. The motors on RF lenses are supposed to be much quieter.