Catalyzes the addition of sulfite to phosphoenolpyruvate (PEP) to yield (2R)-phospho-3-sulfolactate (PSL) (By similarity). Is probably involved in the biosynthesis of L-sulfolactate, which is a major constituent of sporulating cells and mature spores. (2R)-O-phospho-3-sulfolactate = H(+) + phosphoenolpyruvate + sulfite Belongs to the phosphosulfolactate synthase family.