Endnote numbering sequence

I have a 400+ page document with a huge number of endnotes needed. My numbering sequence in now up to  “aaaa”! Is there any way to customize my own numbering process, such as “1a, 1b” etc.

By endnotes you mean the footnote/endnotes inserted by the Word program?  I don’t know, some word processors may give you those options, but that isn’t handled by endnote, and I dont’ think Word has many options like this.  .  .  

If you mean a numbered style with Endnote bibliography, then you can change the settings in your output style to use grouped settings as shown in the attached image.  But these are conventional “endnotes”  – this produces an independent bibliography and if you cite a reference again, it will use the original number and not a short form footnote including “ibid” for example.