It is very common for women that give birth early to have trouble producing milk. There is nothing that I am aware of that you can do during pregnancy to increase milk production. My best advice would be to talk to a lactation consultant.
You really can't do or take anything for milk production while pregnant. My son was a preemie and a nicu baby he's 3.5 months old and exclusively breastfed. I wasn't able to feed my daughter as I got very sick post partum.
Make sure you drink a lot of water, eat oatmeal, and put baby to breast or pump every two hours. If you still don't have enough milk, you can take supplements like fenugreek or mother love.
Be sure to be open with the medical professionals you are working with (OB and people at the hospital when you deliver) about your intentions. If you do end up having the baby early, ask for a pump if your baby is unable to nurse right from you. Do as much skin to skin time as permitted. I found a lot of help from the nursing staff at the hospital when I delivered. Best of luck!
While you are pregnant you cannot take anything to increase milk supply. Even stimulating your breast for milk can cause contractions so don't start early. It took several days for my milk to come in with my first. Maybe after delivery immediately start pumping? This should stimulate the production if milk. At first breastfeeding was awkward and frustrating for me....can't wait for baby number 2 so we can have our cuddle time!