You'd need a spectrophotometer to create a custom color profile for best results. You can still import any profile you currently use to see how well it works for you. Just find where Onyx saves the ICC color profiles, right click the file and select, "Install Profile" to make sure it's installed in the
windows profile system. Then, in Illustrator, go to Edit, Color Settings, and choose the profile for the CMYK workspace. Then anytime you select a color that's out of gamut, it will warn you and let you know it may not print how you think.