Do you already have fish in your pond? If not, and these will be the only ones, it usually isn't necessary to QT them. But if you are going to add these to waters with fish, yes.
I'm not going to answer exactly HOW you should treat them, 'cause answers vary so much. The main thing is that you want the new fish separated for a period of time [an incubation and observation period] that gives you a chance to make sure they aren't carrying anything that would harm your existing fish [bacterial infections, parasites......]. I QT for a minimum of 4 weeks. Some go longer, some less.
Some treat their water; some add salt; I don't do anything special. But you do want to be sure you are checking water parameters in your QT just like in the regualr pond, esp. since the fish will be in a smaller space [wastes will build up faster]. I have a DIY trickle tower filter. I keep a few fish in the tank all the time to be sure the bio-filter stays active. Then I just do water changes as needed. Many folks will just do frequent or daily water changes to keep the toxin levels in check and not bother with having a "mature" filter. Either way works. But remember that the more fish in a smaller space means very frequent water changes!
The question of "feeder fish" comes up a lot. I started out that way, and the few that I ended up with turned out to be very nice fish! Feeder fish tend to come from crowded tanks which makes them more prone to illness. Not that they will be, just a risk factor. I have adopted the policy that I only put fish in my pond that I want to keep -- not expensive, but nicer fish. But hey, if you're doing this as a way to "rescue" a few fish, then go for it!
