References numbered by alphanumeric in addition to regular numbers

I would like EndNotes to use a numbering reference style that can number references by alphanumeric, that is, a, b, c, … z, aa, ab, …; rather than be limited to 1, 2, 3, … or 1a, 1b, 2, 3, …

It doesn’t seem like this would be that difficult to do, and in the government group where I work this is the standard for our reports.  I can do this in the Word EndNote feature, at it seems to me that the EndNote feature should be at least as flexible, and I would much prefer EndNote X6 for my bibliography rather than word.