The thing is, I don't think you can simply add a separate cab profile to a preamp only profile within the same Kemper.
If you make a preamp only profile, that doesn't mean the Cab section is left empty, ready to receive a separate cab profile. At this moment, there's no way to tell the Kemper you're only profiling a preamp instead of the complete preamp/power amp/cab/mic chain. So part of the sound of the profiled preamp will be assigned to the cab section. That's why Viabcroce said to leave the cab simulation on when using a preamp profile.
Sure, you can take the preamp profile you made, call up the Cab section and scroll to another cab preset, but part of your original preamp sound will be replaced because the Kemper doesn't know that it doesn't contain a cab and will try to subtract what it believes is the original cab portion from your preamp only sound.
That's all theory of course. You may like the result anyway, and in the end that's all that counts.
Running your preamp profile through an external cab (real or simulated) instead of a cab profile inside the Kemper seems to be the best option. Or, you could always use a second Kemper to provide the cab section.