First, I know that the upcoming Nexus 3.1 will offer migration of v2 to v3. I also just applied for the beta migration for v2 to v3 (google forms).
My timing sucks, since the next few days I am going to setup and integrate a new Nexus 3 server.
Is it possible to manually copy my old Maven artifacts (from Nexus v2 server) to this new Nexus v3 server and let Nexus v3 reindex everything? If so, how?
It's not possible to manually copy, however Nexus Repository Manager 3.1 is now released and you can use it to Upgrade.
As well, the incredible, edible Manfred Moser created this: https://github.com/simpligility/maven-repository-tools/tree/master/maven-repository-provisioner which you can use to take something from a file system into a Maven Repository AKA Nexus Repository Manager. I haven't used it myself, but it's probably exactly what you are looking for.
For those still checking this out, you can also use this simple bash script to import artifacts into Nexus Repository 3: https://gist.github.com/DarthHater/a4f2738e3bd40d242db22633b59dfd63