Specifically I want to know if I can use seimitsu balltops on sanwa JLW?
Never tried it, but this site (pretty well regarded) says it is possible:
"Happ/IL joysticks generally come with bat tops, while Sanwa and Seimitsu joysticks usually come with ball tops. Sanwa and Seimitsu tops can be changed to bats simply by screwing on a different one of their available tops (and an adapter), while Happ/IL tops require a new shaft all together which a few independent modifying companies produce.
Sanwa and Seimitsu ball tops also come in 30mm, 35mm (standard), and 40mm diameters. Tops made by each company will work on both company's joysticks. Sanwa and Seimitsu joysticks (excluding the JLW-U) require an adaptor to fit Sanwa 40mm and bat tops. "
http://www.slagcoin.com/joystick/attributes_brands.html