This may be possible with a third party APS-C lens, but a Canon EF-S lens will not mechanically fit on an EF mount. The question is "Why?", it is so much easier to add vignetting in post.
Also pay great attention to Peter's post.
Don't even TRY to put an EF-S lens on a FF body. You can damage the mirror mechanism because the lens protrudes in the main body of the camera much more than an EF lens. I have seen people break their mirror in trying to do so.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
04/20/2023: New firmware version 1.4.1 is available for EOS R3
03/30/2023: New firmware updates are available.
2/07/2023: New product announcements!EOS R8 EOS R50RF-S55-210mm F5-7.1 IS STMRF24-50mm F4.5-6.3 IS STMRF15-30mm F4.5-6.3 IS STM