They did it with Wii Motion Plus and Wii Remote Plus and there is no reason why they wouldn't do it with the 3DS.
Except that the wiimote is not a console unlike the 3DS. Want it or not, every Wii owner could just buy a motion plus to use in their wimote or buy a wii remote plus, without needing to buy a new Wii.
In the case of the 3DS, your option would be buy a whole new console if you want the extra built-in circle pad or buy a Circle Pad Pro if you wanted to keep the "lite" version of the 3DS. As it was stated before, the CPP doesn't work like a built-in circle pad, they're not compatible. We could have a situation where there are games that only work with the CPP and other that only work with the extra built-in circle pad. It would be bad for developers, consumers and Nintendo themselves.
The PSP didn't have double analog sticks too and I don't see PSP owner crying about that