Yes, I'm also a foreigner, converted my license in November last
year.
Just called TP HQ, the officer told me that in order to convert,
the driving license must be at least 6 months old since the day it
was issued. But the person doesn't have to stay outside Spore for 6
months. So he can get the license from his own country, then enter
Spore and stay happily here for 6 months until it's valid for him
to convert.
I'm not saying your information is wrong, but maybe the regulation
has changed over the time. Or maybe your case was a special one,
who knows? 
Anyway, warlock, since there're different versions of the story,
I'd suggest you to call them personally to verify about the
procedure.