After the death of Victor Garber's Martin Stein/Firestorm and the subsequent departure of Franz Drameh's Jefferson "Jax" Jackson/the other half of Firestorm, there are quite a few vacancies that need to be filled and Lonsdale fits the bill, especially since he's been away from The Flash for Season 4 so far.
“We loved working with Keiynan in our series premiere — the episode in which we learned that in between Seasons 2 and 3 he had been fighting crime in Central City with Nick Zano’s Nate Heywood. So when his character stepped away from Flash to go on walkabout, after being dumped and feeling somewhat estranged from his family and team, we knew that Wally West would fit in perfectly on Legends,” said executive producer Phil Klemmer in a statement. So now, Wally West is not only back in the company of old friends from the Arrowverse, he’s fighting alongside a former enemy and career arsonist Mick Rory/Dom Purcell. By the end of this season, Wally will learn that the Legends have a different definition of what it means to be a hero.”
While more at home in Barry Allen's universe, Wally will reportedly not return to The Flash before he debuts on Legends. He'll make his first appearance in Episode 11 of the show's third season on Febrary 19 and will officially be made a member of the team in Episode 13.