@Flexo55 Sure it is possible. Just think about your requirements.

Which input formats (16:9, 4:3, 5:4, ...) and resolutions (1920x1080, 800x600) should be accepted on the HDMI side? Which output standard should be supported? (NTSC/PAL or both?) Which framerate you need for your CRT (50Hz/60Hz/...)? Shall it support conversion options when converting e.g. from 16:9 (in) to 4:3 (out)? Do you also want HDMI audio looped through the device or will you use the Pi's headphone jack or USB audio? Do you want extra filters supported by the device, e.g. blurring, scanline modification, ...?

This might be one device as example: https://www.startech.com/en-ch/audio-video-products/hd2vid2 (obviously you need a Micro HDMI to HDMI adapter)

I would also ask or research in some AV forums on recommended products for your project and your requirements.