What is the best way to add new field to an existing Primary Key?

 Hi All,    I have a situation which need to add a new field to an existing primary key. The table rows is around 3 millions. The only way I know to do this is

- drop primary key

- re create the primary key with adding new field

but my concern is that the rows contain 3 millions rows. Is there a way to estimate how long it will take to perform this changes ?

Rgrds

Ezree