Doing this conflicts with the design of dependency management systems, and may be a bad idea, but it can be done. There are a couple of methods, depending on your situation.
If you have control of the repository, or are the person deploying the artifact, you can turn on the "unique snapshot versions" feature, which will give you exactly what you're looking for. (Take a look at http://maven.apache.org/pom.html#Repository)
If you aren't the one deploying, and don't have control of the repo, then yes I suspect copying a specific version of the artifact, possibly with a unique name, version, or classifier, might be your best option.
Caveat: All that being said, this seems like a bad idea -- the SNAPSHOT version is supposed to be the latest and greatest that passed all its tests, and shouldn't be treated like a released version that never changes. It seems like if you want a version that's not going to change, you shouldn't be using SNAPSHOT. But only you know your use case.