Yes. In fact that's one of the best times to use it. Hopefully the plant will be able to absorb enough of the hormones to stimulate some new root growth.
I use Megathrive, bit only when repotting.. it's great and you'll see a boom of new growths!! It has a lot of components that accelerate the metabolism of the plant.
I buy it here: Megathrive. But I'm sure it's sold in other places as well... I like this store, because it ships internationally also, but check your local gardening supply store, they might have it .
I use KLN when I water my orchids. I soak them in a huge sink with water and the correct amount of KLN for 15 minutes every other time I water them. I only do this for non-spike plants and especially ones that are a bit worse for wear. Once I see roto growth I back off the KLN to once a month.
Yes, using a rooting hormone when repotting can be one of the best times to use it. Some may advise that you wait a day to water after repotting to allow the newly-cut ends to callous and help avoid any potential for infection. I personally water with Maxicrop (a seaweed-based rooting stimulant) about once or twice a month in the summer months and maaaybe once a month during the winter (if they are looking a little sad only during their resting period).
Just be aware that a rooting hormone which is too strong will actually inhibit new roots rather than promote them. What works on non-orchids is often too strong for orchids.
Powdered hormones usually don't work well for orchids. Liquid ones that you can dilute to a suitable concentration are best.
KLN is reported to work well on orchids. Seaweed extract is also something that can help if you get one with the right hormones present. Kelpmax is a seaweed one that Ray has tested and now sells because he liked the results. I'm pleased with the one I've found in the UK as well.