I never ran DLC content on my psp, but I think all you need is a plugin called nploader.prx (googling gave me quite a few sources to DL it) and just have the NoDRM Engine option enabled (disabled? oh well, you can test which one's right
) in Recovery Menu > Advanced. Apparently, the latest version of nploader.prx allows the user to use custom locations, but it's recommended to just place the DLC file into PSP\GAME.
EDIT: forgot to mention that the above is only for
PRO; dunno about other cfws (GEN, M33 or ME).
EDIT2: I assumed that you're
not paying for your DLC.
As for substituting music files, I've no experience what-so-ever at this so I don't know. There's a plugin by Plum called
mp3 player plugin that allows you to play music files within your MUSIC\ directory (it also reads subdirectories, so it'll read files within folders as well) while on the XMB or playing games (I don't think it supports pops). It has the ability to mute certain (or all) channels while playing the game, meaning that you can rid the background music, character dialogue, sound effects, etc. (or mute everything) while having the plugin active. The button combination is editable to fit your preference.