There isn't, you can only get the Id of a list of objects easily by converting the list to a map and getting the keyset. (See [I just want the IDs?][1])

I'm afraid you're stuck to looping over the list, and adding them manually like you are doing now.

  [1]: https://salesforce.stackexchange.com/questions/9331/i-just-want-the-ids/9332#9332