I actually tried something like that with a previous pair that I tried to make, but I didn't like the way they looked. The flap didn't really lay over the fingerless part correctly, and there was a pretty wide gap between the cast off and the mitten flap, and the only way they laid flat was to attach a button. So I thought picking up stitches a few rows down would ensure that the flap would lay over the fingerless part instead of just barely meeting, does that make sense? The "mittens with a flap" pattern actually says to knit a row of purls a few rows before the bind off, and then you pick up those purl bumps for the mitten flap.
Round 11: k20, p20 (The purl bumps mark the placement for the flap on the back of mitten.) Round 12: Knit around plain. Round 13: While decreasing evenly 4 sts around, work k2, p2 ribbing. Rounds 14-20: (k2, p2) around Bind off loosely, and cut yarn.
Flap: Cast on 18 sts to an empty dpn or the cable needle (if magic-looping). With same needle, and the mitten back facing you, wrist ribbing at the bottom, pick up and knit a stitch in 20 purl bumps on the back of the hand. Join and arrange these 38 sts to work in the round, 18 free sts, 20 sts on the mitten back.
Just want to make sure I'm understanding that correctly.