Вы можете использовать ContextShift.evalOn :
def backGroundTask = IO {
Thread.currentThread()
}
val contextShift: ContextShift[IO] = IO.contextShift(ExecutionContext.global)
val ec = ExecutionContext.fromExecutor(Executors.newCachedThreadPool()) //create other execution context
println(backGroundTask.unsafeRunSync()) // will print Thread[main,5,main]
println(contextShift.evalOn(ec)(backGroundTask).unsafeRunSync()) //will print Thread[pool-1-thread-1,5,main]