Short answer, no, you can't add stamps to Snagit on Mac... yet. It's certainly an area we're looking at.
Here's a little more information and a work around for the stamps issue.
So, first, a little more about Snagit and stamps. Stamps don't work exactly the same on Snagit for Mac as they do on the PC. For example, stamps on the PC are standard raster, bitmap images, usually a PNG file. But, on the Mac, Stamps are vector graphics. What this means is that Snagit for Mac stamps act a lot like callouts. You can resize the Snagit for Mac stamps and they'll never look pixelated or blurry. On the PC, Snagit stamps will get blurry and pixelated if you make them too big.
Why is this important to know? Well, because Snagit for Mac stamps work a bit differently than Snagit on PC stamps, we couldn't use the same stamp bundles and methods for both platforms. And, like I mentioned above, we're looking at all of this and things will change in the future. Snagit on Mac is still a 1.0 product.
Of course, I mentioned a work around, and here it is: