It's illegal to offer. If you ask, they can do (if so inclined) because it's you breaking the law, not them.
That, of course, is assuming they are prepared to do that.
I have had a couple of massages at Jasmine's; the first when it open just after Easter last year and the second when I was back working in that area in October.
I never ask or accept extras. I do, however tip well for a good massage.
Both times, they received reasonable tips.