SMS 2003 - Tools: Collection Adder
This is a very handy tool written by Mark Nunn will allow you to take a text list of hostnames and add them to a collection. One of it's better features is that you enter in your site server information and it will query, then list that server's collection in the window below, making it much easier to find the collection ID.
Enter in the collection ID, then the path to the text list and bingo!
As a side note, I generally make a new collection for these types of tasks, since they are generally "special order." That way I don't have to manually delete the machines from the collection properties later... just delete the entire collection.
http://myitforum.com/inc/upload/8510Collection%20Adder.zip
Enjoy!