Unlikely tt pumping will cause a decrease in supply. The opposite will instead likely to happen, think of demand and supply. The more you pump out, the more your body will produce as it thinks tt ur baby needs it. While it's good to freeze whatever u have now for later use (esp the colostrum as it has alot of good antibodies for newborn), do be careful as pumping and stimulation of the breast may cause contractions in the later weeks. So best to do it aft 37th wk because if u get contractions then, at least ur baby's lungs are better developed and ready for child birth.
Will advise to check with your gyne. Because now you already 31 week might get contraction while pumping now.