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.
No, you'll have different passwords for every single site you use. The thing is the One Master Password is the Key to "Constructing" the necessary passwords for sites you "generate* with masterpassword.
Master password doesn't sync at all between devices. Its an extras step but you'll just need to add each site to your device to generate the same password.