It was told by Gan Fall most people (still few) came to White Sea through an alternate way to the Knock-Up Stream. How much is known about this method?
As per One Piece Wikia
The only way that has been shown so far is the unorthodox and dangerous way in which the sailors have to take a Knock Up Stream. Another way, mentioned by Gan Fall is the High West, as well as a couple of unnamed islands. However, these routes are apparently very treacherous, as Gan Fall states that, "If you had brought 100 people, it would be a gamble as to how many would survive". The section first appeared in Chapter 237 and Episode 153.
It's highly possible that Bellamy made use of the High West Route given that everyone in his crew except him died, but nothing except what's stated above is known about the other routes to reach the White Sea.