How do you plan to add the plug-in capability?
As far as I know unless you do it yourself, there
is no company right now that is doing it for
consumers - only for fleet buyers.

There is a company called Hymotion which will
do it for consumers from 2008 but initially they
will only offer for the Prius and the Escape Hybrid.

Not sure if they will do the Altima Hybrid ever as
the numbers are so few.

It will offcourse void the warranty.

Plug-in is still atleast 3 years away.