[Porsche-O-Phile]

Agreed although it might be allowed by code in some areas - I'd double-check (think seismic requirements) and in any case I'd get a little leery of anything higher than maybe 3'-4' unreinforced, especially in a seismic zone.

I tend to agree with most of what's been said above regarding pricing and construction. Depending on how the existing wall is laid out and what the top condition is, it might be a simple matter to grout every other cell (or every fourth cell, whatever it takes per calculations) with a bar in there. Essentially cost of material - if there are capstones that have to be removed they'd likely get pulled off anyway to extend the wall up higher so no increase in labor cost, just more grout and more (or longer) bars...

What's the goal anyway - sound attenuation? Visual/light barrier? Deter trespassers?

How are the mortar joints on the existing portion of the wall? Without reinforcing that's your likely failure point, especially with new load above it (with fresh, newer, stronger mortar joints).