The easiest way to do this will be to download ffmpeg and create a batchfile. You should be able to drag your 6 wav files onto the batch file icon and have ffmpeg output a 5.1 wav. I’ll double check this before I post an example batch file.
Edit: Another option is to use a program called wavewizard to merge the 6 files into one multichannel output. You have to be careful about files larger than 4Gb as normal wav files won’t natively support larger file sizes.