Using a letter to identifiy citations (in-text and bibiliography)

I am trialling Endnote X9 with a view to converting our organisation to this rather than MS Word built in cross referencing.

Our in-house style uses letters to identify the citation. For example “in accordance with reference A, the manufacuturer …”. The corresponding bibliography entry would therefore be:

A.   RUAG Technical Terms of Delivery - PT76 Primer, version 1.0.

I can create output and citation styles to give:

“in accordance with reference [1], the manufacuturer …”. and a bibliography entry of

[1]   RUAG Technical Terms of Delivery - PT76 Primer, version 1.0.

But it appears that there is no way to create citations that are lettered (A-Z then AA, BB etc.)

Is there a simple way to to this or is it possible to add this feature?

Many thanks for you assistance