Sorry, but there's no downloadable content for DDR Hottest Party 1 or 2. You're stuck with the songs you have.
It gets worse! There are only three DDR games that can play on the Wii: Hottest Party 1 and 2, and Mario DDR, which is so rare it's virtually unobtainable without a big wallet.
To expand my DDR playlist, I actually bought a last gen console and several cheap used DDR games for it. I also purchased $20 generic universal pads, so I can use them on any console new or old.
I realize buying a whole console for the sake of DDR could be a bit extreme, but since it's a major source of aerobic activity for me through the winter, I consider it worth it. I purchased a PS2 to play on, so of course now I have a system that plays a staggering amount of fantastically good games. Needless to say, I don't regret the decision! My sister has an old xbox, and that has a wealth of DDR games as well. Somehow Konami skipped the Gamecube, though.
I'm not up on the 360 or PS3 versions of DDR, but none of the other versions have songs you can download. Not one. The good news is that some complete games are only $14 used, and there's a program for PS2 called Dance Factory that will make DDR routines out of any music CD you have, bought or burned. That means you can DDR to anything you want if you put in a little effort.
Sorry about the DLC, because I'm in the same boat--but there are plenty of reasonably-priced options out there if you're willing to go old school.