I agree with both of the responses. Richcopy/Robocopy/Securecopy all are the recommended tools to use. Just that you need to pay for Securecopy and also with this you could move the local groups as well, if there are any.
NDMP backup/restore won't be any easy method, as you won't be able to leverage the NetApp's native ndmpcopy. You would need to go down the 3rd party ndmp backup/restore tools.
DFS is a good way of minimizing the pain in the migrations if there has not been one impletmented already, you could consider that as well. But with DFS, you'd still need to use Richcopy/Robocopy/Securecopy types of tools to do the migrations.
One other important thing to pay attention to, is the security style of the file system. Make you have isolated the data on the source to either NTFS or UNIX style filesystem and then migrate it to NetApp in their respective security styles. Don't play with mixed security in such migrations.