You can use the Channel Mixer to do cnk's suggestion with more control. In the past I used the color settings to go from rgb to cmy (without k) but I can not get it to work in the new color management settings. I remember it was something one had to set to default to not change images to black-less ones... An internet search on PS cmy did not bring a lot of hits either, but I learned a log of inkjet printers use the cmy model. Doing it by hand and saving the channel mixer settings for later reuse might be the best option.