We don't offer that feature. Please note that the objective of Master Password is not to be a password vault. As such, it is not very useful to import existing passwords into it.
The objective of Master Password is to generate new passwords for all of your sites based on an algorithm and your master password, so that you have strong unique site passwords that do not rely on storage and can never be lost.
As such, adopting Master Password normally involves changing your sites' existing passwords to the one given to you by Master Password.