Not a genius by all means, but you should try this :
Import custom model (a) into Daz at the correct scale
Load a g8f model (b), and morph it to get (roughly) the same proportion
Send both in Zbrush via bridge at base level
Project (b) into (a), export back as a morph for (b)
You will need few tries, ears gonna be problematic from what I see (you can up subdivision for better projection but don't 'apply' it).
Shading part should be the easiest part tho.
After downloading the file, projections gonna be mostly messed up, and just too much work to do it.
Maybe isolate just the head and make a morph out of it and redo similar body via Daz.
Someone may have a better solution.