@fnddoctor I can't tell you how to add glycerin to your CP soap as I've never done that.
Can I ask why you'd like to do this? Is it that that your hoping it will result in a more transparent soap?
Out of curiosity, I read some old threads on this forum where the addition of glycerin was discussed. Other than one person saying that they mixed their colors (micas?? clays??) in a small amount of glycerin, most other posts were pretty lukewarm on the idea- and commented adding too much could give the soap a sticky feel. Apparently, glycerin is created during the saponification process, with your finished bar of soap containing 7-10% glycerin.
So, anyway, I'm wondering why you want to add the glycerin. Maybe that would determine WHEN or how you might add the glycerin,