Hi Marco,
Dropdowns are sorted alphanumerically and unfortunately, you cannot specify otherwise. We understand that this is not ideal and the ability to sort dropdowns has been included in Method New and will be available when we launch.
How are you building the filter? Have you tried using the Build selection list option? I believe this option would be ideal since, as I understand it, you have a specific list of account numbers to be included.
I hope this helps.
-Audisho