It's a closed shop, so you can't, no.
Thing is with unions in this country is that they all eventually get greedy and kill the goose that lays the golden eggs. It'll happen with the RMT sooner or later, at which point several million commuters will laugh their heads off.
The premises for their strikes are invariably irrelevant. They'll always find a reason simply in order to flex their muscles and remind everyone who's boss. The point is that the whole thing needs to be broken and started again with workers being paid sensible amounts, with sane levels of holiday and benefits. There is no more disgraceful example of what happens when unions are afforded too much power and control than the London Underground. They single-handedly demonstrate everything bad about the union movement in the modern world.