EA doesn't own Respawn and Frosbite is only for EA's internal teams so no, they don't have easy access to Frostbite engine. One could say that maybe they should look at UE4 for future titles, but there must have been reasons to pick Source in first place.
They said that the main reason they picked Source for the first game was because of it working well with PS3 (since that was the most problematic platform last gen).
But things took a turn and the PS3 version got cancelled and by that time it was too late to change engine.
http://www.eurogamer.net/articles/digitalfoundry-titanfall-tech-analysis