I don't routinely feed my horses sweet-feed but do keep some on hand for mixing-in medicines or wormers.
For worming, I take 1/2 cup of sweet-feed, mix in the wormer and add a table spoon of pancake syrup. Stir it up real good and serve...
They love it.....and none gets wasted.
I find that much easier than trying to squirt it down their mouth.
So do you just take the normal stuff that you would squirt in their mouth and put it in the feed?
I'm sorry that I wasn't more clear on that but yes.....I just push the plunger and squirt the entire tube-full into a bowl......then stir-in the sweet-feed and syrup.
A half-cup aint much, so sometimes I just give it as a treat. Other times, I'll mix the mixture into their regular (pelleted) feed and give it as part of their feeding routine.
It don't much seem to matter which way ya do it. The main thing is that they are getting it and none gets wasted.
There are also some liquids specifically for top dressing, but I really don't think those are much different than just squirting a paste dewormer onto food like others have mentioned.
The only natural dewormer I've run across is "worm check" (http://www.chamisaridge.com/search/1/7901). I've never used it, but looking at the ingredients it may be effective, though probably not as much as chemical dewormers. You can also try feeding things like all-natural unfiltered apple cider vinegar (Bragg brand is good) and garlic on a daily basis, but again it's not going to be as effective as chemical means.