I would imagine you could use advanced file selections and use includes.
In job properties, click on the folder you want and then advanced file selections. In the path you should see your chosen folder. In the file field just below the path, type in you a*.* and make sure the Include option is selected. Hit ok, and all those files will be selected. Just keep repeating for each letter up to M you wish to back up.
I am not sure if there is a way to enter it all at once, but it shouldn't take you too long to create those include items.