Since Lauren Ward is American, there's no reason that she couldn't come to New York with the production if the Roundabout wants her and she wants to do it.
In fact, it probably makes sense to bring her along, as one more element of the successful London production in place for the transfer.
I wonder, in the case of someone like Ward, a well-regarded actress, but not a star, if it would be more cost effective to bring her over or to hire locally? The local hires don't have to be housed, but would require more rehearsal time and require new costumes and wigs, etc. Ward might not even need to be housed, if she still has a home in NYC. But she would need to be brought over and paid a salary that makes it worth her time to be away from home for several months or however long this run will be.