Is it possible to produce milk or colostrum if you aren't pregnant?

Yes. Many women who have had children and have breastfeed will go on having small amounts of dried colostrum that appears on the nipples. If there is a larger amount of discharge, then it is necessary to see your doctor to check for other issues that could cause lactation or nipple discharge .
Yes. Milk production can be triggered by nipple/breast stimulation and has been used to allow adoptive mothers to breast feed. The success is variable.