Porsche did build 4 964 Turbo convertibles, 20 964 Speedsters Turbo-Look, 2 993 Speedsters, and even 14 993 convertibles with a 964 Turbo engine inside. If you pay enough, they will build it.
As TargaTim said, it's just about swapping the right parts. PASM would need new settings, though.
Unfortunately, I suspect it'd be cheaper to buy a stock Turbo and a stock Targa than to get a single Turbo Targa.