He has voiced characters like Green Arrow and Leonard Snart on animated DC projects in the past, so it was possible he would reprise one of his past roles.
Diedrich Bader is reprising a role himself, having voiced Batman on The Brave and the Bold.
The news that Alan Tudyk would be part of Harley Quinn was actually confirmed weeks ago with the announcement that Kaley Cuoco of Big Bang Theory fame would voice Harley herself, but there were no details about who exactly he would be voicing.