So I fiddled around today with my need double ended hooks and some yarn. I decided to see if I could start a top down hat worked in a Tunisian knit stitch.
I worked the hat out for about 5 rows of increase and these photos below are the results. I realize I didnt use the best colors for an example work, since I used a white and black variegated MC with a black CC but it was the yarn I had that was handy and easy to unravel if I decided to try working the hat differently if it didn't work up like I had planned.
So here is what I used --
Size M/N 9mm double ended hook
Worster Weight yarn in 2 colors (Mc/Cc)
Stitch marker (I used a scrap of white yarn)
You need to know some tunisian before attmpting this, how to work a TSS(simple stitch) and a TKS (knit stitch)
Here is the basic pattern I started working--
R1- MR sc 8 (work normal sc)
R2- This is where you will leave your loops on the hook-- I did this increase round by picking up a loop from the front, then back loops of the top of each stitch. You should need to work off loops as you add them on, for me I was comfortable having 6 stitch's on the hook at any time. You work them off as you would work a return pass in normal Tunisian with the CC, leaving 2 loops of the MC on the hook to maintain consistent tension. 
R3- Inc, tks (the way I increase was to work a normal tss then in the same stitch work a tks)
R4- Inc, tks 2 
R5- Inc, Tks 3 
Just keep fallowing this pattern of increases till you get the width you need for the person the hat will go to. The Tunisian knit stitch has a natural curl to it so dont worry when it starts to curl out wards, once you start working the length it should work itself out.
A little math to help you out--
What that means-
C= the circumference of the head
R= the radius you need to get to insure your increase row will leave you with the circumference you need.
So lets say you want to make a hat for a toddler with a 18 head. To make sure you work the right amount of increase and dont make a hat to small or way to large we need the radius (that is the length of the start to the edge of the hat ((or from the center of the circle to the edge))).
So 18=2piR we divide both sides by 2; 9=piR; and now we divide both sides by pi (3.14) so we get 3=R (I rounded up the real number was 2.86). So from the start of the hat to the edge needs to be 3 inches (or 6 inches across from edge to edge) in order to be wide enough to fit.
If you want the math for the diameter (from edge to edge) its C=piD so that math would be 18=piD; divide by pi D= 5.73 which I would round up to an even 6 inches.