The Belkin Rockstar will work but I would not use one for mixing as it is only 32ohms, you can bang screws in with a hammer too but it is not the proper way to do things.

Either make your own or buy a proper pre-made passive mixer, passive mixers are typically 10kOhms BTW

Shameless plug - I make a small passive mixer for exactly this task, £15 plus postage to forum members, details here: http://operator-1.com/index.php?p=/discussion/1019/swiss-army-knife-for-pocket-operators-op1-volca-modular-etc/p1