1. Locate the following CSS file in your XMetaL program files directory:
{XMDIR}\Author\DITA\XACs\ditabase\ditabase-base-override.css
2. Save a backup copy of the CSS file and open the original in an editor.
3. Add the following at the end of the file:
[class~="topic/ol"] [class~="topic/ol"] {
list-style-type: lower-alpha;
}
4. Save the CSS file and then reopen your XML document in XMetaL (or switch views, e.g. switch between Normal and Tags On).
NOTE: This should give you alphabetical items for any ordered lists that happen to be descendants of any other ordered lists, regardless of what other elements may be present. I have not looked at the behaviour of the default PDF stylesheets, so I do not know if this is the exact same behaviour.