Miles expire based on the activity of the account. If you transfer them to your (active) account, these miles would stay active as long as your account is active. Unless there is some exception for transferring miles, but I would doubt that.

Having said that, I value FB miles at about 1 cent each. Therefore I would not personally be willing to invest 120 euro (the value of a RT ticket within Europe!) into 10K miles that will likely not even be enough to get a similar RT out of them.