I believe the login music can be changed with a dropdown box on the login page (top-right). There should only be a setlist that you can choose from, you can't import anything specific, the ones that are there are the ones you have to choose from. All tracks should be unlocked for every player.
I hope that answers your question, or if I'm reading them correctly.
^ The drop-down menu is just for the login background, not the music (if I recall correctly, I just have it muted).
This page lists all the music that is/was used at login: https://runescape.wiki/w/Login_musicCreator of the quarterly M.U.P.A.Q. and I.B.A.T.A.W.A.T. Awards™. Do you notice unhelpful but non-trolling posts? Or someone asking a question for which the answer was already given? Nominate them here!