Very interesting. Cannot help with any advice / knowledge here but it would be nice if you posted your experiment details and results. Maybe some of could then get aboard this, try our own and share the results.
Well, I have no exact receipes that I could publish. The most important (obvious) advice is, that you can always add some more incredients of your choice but not the opposite way. If you are unsure it is recommendable to decant a small amount of your After Shave in a separate bottle before you ruin the whole thing. Some incredients should be added carefully, i.ex. menthol crystals because even a very small amount of this stuff has already a strong effect.
Then you should be aware, that some incredients can not be mixed and will disperse again after some time. For example menthol is soluble in alcohol and oil but not in water. For adding oils in After Shaves you need some kind of emulsifying agent, if not already present within the original After Shave itself.
Then it is advisable to get some information about which incredients are already present within the original mixture (INCI = International Nomenclature of Cosmetic Incredients). And you get an idea about what is missing and could be added to get an individual improvement for your personal needs.
For example, if you take a look at the incredients of Myrsol Agua Balsamica, you will notice that it contains:
Alcohol denat, Aqua, Rosmarinus, officinalis, Thymus vulgaris, lavandula latifolia, sorbitol
Obviously there are some useful herb extracts inside but except sorbitol nothing to help against dry skin. Therefore I added some Aloe Vera gel and some Panthenol and now it is much better for my skin. But as always YMMV.