Yes, that's what I first tried -- and abandoned. The mando neck is too narrow for the chording a 'uke demands, at least with my not-small hands. I measured neck widths at fret zero: Kohala soprano 'ukes, 35mm / 1.4". Kala tenor 'uke: 38mm / 1.5"; Kay mando: 30mm / 1.15"; Rogue mando, 27.5mm / 1.08". Yup, too narrow.
I have a backup plan. The Rogue sounds sweet (to me) in Irish tuning, GDAD. Yes, I play open a lot. So I'll leave the Kay in concert tuning so I can properly learn the bugger, and keep the Rogue in Irish (for now) for fun. My other plan, tuning the newest Kohala soprano 'uke as a mando, was easy. The set of Aquijla fifths nylgut strings cost a whole ten bucks shipped and they sound nice and bright on the 'uke.
So if I want an 8-string 'uke I'll just have to buy one, not convert a mando into one. And those I've tried at the nearest music store didn't impress me. Well, I'll be in bigger cities soon, haunting larger music shops. Maybe I'll get lucky. Or maybe I'll find a fine mandola that will drive my wife to say, "That sounds great! Let's buy it!"
If you have small hands and thin fingers, go for it! But I have another idea for reviving and mutating a problematic mando: Remove two strings and replace the nut and bridge with six-slotters to turn it into a three-course axe. String spacing will be about the same as a soprano 'uke; lots of room for fingers to fly about. Or leave the strings and cut the nut and bridge for two 3-string courses and one 2-string course. See
this huge set of instruments and tunings for possibilities.