The sleeves :
A few things to remember if you are doing a similar thing:
-add 1/4" seam allowances to all seams
-add about 1 1/4" to the bottom of the sleeves for the sleeve hems
In the end it was a very easy and quick process. I just traced the front, back, and sleeves of the tshirt from the bust up and added it to the maxi dress pattern. You probably wouldn't even need the maxi dress pattern. You could just compare the body of the dress to an existing dress that you have and draft the bottom too.
I saw that Mimi posted an online class to do a similar thing for a tshirt maxi dress a few days ago. So if you don't feel confident winging it this might be a really great option. I am sure it's a great class.