I don't think there is that option, and it wouldn't mean anything --- Mac OS 8.X doesn't use associations that way. Just add the suffix in Mac OS X, Windows, or Linux.
Yeah I can't imagine why you would need to do that in Word on a Mac. Just put the file on a CD, floppy or on the Internet and then rename it on a PC or Linux, and make sure the file is compatible with the PC version of Word...
Thankyou. It's just because I needed it to put something on fanfiction.net, within a deadline, and I thought I would be able to as there were already some files on my computer with ".doc" on the end, but I guess they must of come from somewhere else. (I deleted what was on one of them and C&Ped my thing into it in the end.)