Clearly Asker123 the automatic feeders are for the majority of softener users i.e. people who have busy lives and would otherwise forget to do the ResCare. Of course, such folks could also fail to notice if the auto-feeder wasn't working or if the ResCare bottle had run-out.
So the decision is yours to make. Absolutely yes, you can add it manually, and the best place to do so is where the float assembly is because then the ResCare goes into the water that is drawn-out to clean the resin. That's what I am doing, and I also splash a cup of fresh water after, both to clean the cup I'm using and to wash the ResCare away from any float assembly parts it would have touched, though I feel certain leaving such would not harm anything.
FWIW I have a Reminder on my iPhone to check the softener about every week or ten days and try to add the ResCare the evening before it regens. More because I'm anal than that it would "go bad" if you put it in earlier, even right after it regenned.