Import mp3 into FF6?

I want to know if is possible to import .mp3 into ff6, i don't know much about ff6 sound hacking so...

The average mp3 filesize is larger than the whole game of FF6, so... no. There are several documents on the wiki that go into what is needed for music hacking. I believe there are enough tools to convert MIDIs into the in-game sound engine, but I have not tried to do so myself.

MP3 -> FF6 music format conversion is impossible. As PowerPanda said, there are tools like PetiteMM and TinyMM to convert a 8 channels midi to FF6 MML format, but that just gives you notes and octaves, everything else has to be added manually to the MML file. Then you convert that MML file to FF6 binary song format and insert it in the ROM with another tool like mfvitools. There's also the MSU-1 option, you could install the audio hack and define your own OST instead of using one of the mod preset. It's less work than music hacking but you still have to manually add loop points to your tracks in order for them to loop properly in the game.

You could hypothetically convert an MP3 to a Midi, then that Midi to an MML, and then that MML to FF6 binary. But it's like faxing a fax over and over, the loss would be immense and what you end up with probably would not even be worth the effort.

Maybe mute the game and play your music in another program? Unless you're looking to incorporate it into a hack of your own design, in which case it might be worth going through that fax chain and tweaking accordingly. But it would not be a trivial endeavor.