I think you could use the same concept for a thumb as on these fingerless mitts. LINK In the part about doing the Left Hand mitt go down to right after where it says to work 18 rounds of 4X1 rib. Then it gives some directions in “next round” about knitting to a point and then knitting 7 stitches with waste yarn, slipping them back to the right hand needle and then knitting them again with the project yarn. Then you just keep going until you finish the mitten. This mitten is worked cuff to finger tip, but I think you could do the same thing going the other way.
You’ll have to determine the placement for the thumb. This will be just a basic thumb with no gusset or anything. I think you could make both mittens the same way and they would work for either hand.
When you finish the mitten you go back to the waste yarn and take it out and pick up the live stitches, add a few between the needles and then make the thumb out of those stitches. This is for an adult, and you will have to decide how many stitches you need to allow for the thumb. They started out with 7 on the waste yarn and then had 17 for the thumb when they had stitches from both sides of the 7 and a few more added between needles. Your number will be based on how big you want the thumb to be and the weight of your yarn.
To knit the thumb itself you could decrease it like a hat at the top a bit and run a tail through the rest of the stitches and pull to end off the thumb. Make notes so you can work the other one the same way.