Omg, going against the status quo c:
If the father can learn the HM naturally, and the offspring can learn it through HM, then it actually is possible to give the offspring an HM move via breeding.
For example, if a Female Lumineon bred with a Male Seaking (that learnt Waterfall at Level 40), then the resulting Finneon would have Waterfall.
Unfortunately, in your example Farfetch'd can't learn Defog naturally, and therefore the offspring will not have Defog.
It's also worth noting that this works with TMs as well, which is much easier as many Pokemon can learn TM-applicable moves naturally.
Hope I helped. :)
Multiple parts of Bulbapedia and Serebii that I can't be bothered linking to :P