You'll have to convert to a Scala collection and then back to a Java collection:
import collection.JavaConverters._
val l: java.util.List[User] = ...
val l2 = l.asScala.map(_.id).asJava
// l2: java.util.List[java.lang.String] = ...
Alternatively, you may be able to write your own collection builders and CanBuildFrom
implicits for the Java collections, but this would clearly be more work.