It can be done, but it requires a good understanding of nes assembly language and debugging tools. You're gonna have to reverse engineer the compression format, compress the new background tiles into that format and insert that into the game.
In theory you can write your own uncompressed nametable writing algorythm into the correct section of the game's code, but you're going to run into trouble finding rom space to do it. Unless you're really lucky and this game stores every background already uncompressed, then it would be the easiest scenario, but that's almost guaranteed to not be the case.