I know this is an old thread, but there is a way to open .cpk files, which are compressed CriWare files. I forgot what the tool is called though.
Also afaik, there is no way to rebuild a .cpk file.
For texture editing, if you have Citra (a 3DS emulator), you can dump and replace existing textures, though the changes won't carry over to the 3DS console.
Finally, to bring back a character from one of the older games, you can use a tool called
Savvy Manager to import one of those characters over yours or some of the non-playable ones.
However, if you have Style Savvy: Fashion Forward, you can add the characters from the older games (as well as Styling Star), as extra contacts, though they'll only appear in photo shoots and AR Photos, as well as the profile menu.