Bob,
Yes from what I read not a built in feature, but a mod, and those sound like more than just a bit of work. Your decision on if its worth pursuing.
The other difference with making a post saying "LIKE" and just clicking a button, is that is a new post and raises its visilbity (the LIKE would probably not do that since it isn't a post). That may be considered a good thing by some, but then if every day there's a bunch of threads with new posts and they're mostly LIKEs people may not be thrilled with that.
I'm on other forums where LIKE exists, and it can be a good tool. I'm not against it but feel if it cannot be done easily then we shouldn't do it. I know its more popular in social media and blogs (almost any new site with comments allows them). Its like emojis, just because some places have 500 doesn't mean everything needs that.