You can use the Fusible No Show Mesh which is a cut Away stabilizer. It is soft and will not add bulk to you sweatshirt or the design.
Depending on your stitch count of your design you can fuse one or two layers on the back of the shirt.
2 layers criss crossed at 45 degree angles to each other for a more stitch intensive design fused to the back.
The Fusible No Show Mesh stabilizer will control the stretch of the knit fabric of the sweatshirt while you hoop it and also while you embroider on it.
Then hoop the shirt and float a pice of tear away under the hoop. The tear away will give you design a nice crisp look.
You can also add one layer of the Solvy type water soluble topping on top of the shirt and this will help hold the stitches above the knit fabric while you embroider.
Use an sixe 75/11 or 80/12 Ball point needle.
Leave it to EL they always come through....I didn't realize you need to use a ball point needle, good to know......thanks for a great question.
here is a tutorial from emblibrary. They have instructions for almost everything.