Have KM send Shift-Command-G, then you can just insert text (or a variable) by typing or pasting. Here's how it looks in my MacroBackerUpper macro:
Group.kmactions (6.2 KB)
That's not a full macro, of course, but it shows how it fills in a filename in the Save dialog, then brings up the path prompt, fills it in, and makes sure it's gone before proceeding.
-rob.