I've heard that Procedo do have a DX -> FSA migration tool. Do not know anything about it. You would need to ping them direct for more information.
If you are happy to un-archive from DX and then re-archive with FSA, then DX does provide you with tools to un-archive. They have media tasks and from there you can choose a 'file restore' task, or a 'compact task'. File restore lets be you selective with which files to restore. Compact will restore all files for a piece of media. Check the DX admin guide for more information.
You may find that when the files are restored they still retain the DX extended attribtues (EA's). This is where DX inserts its meta-data. FSA can archive these files OK, but by default it cannot turn these in to placeholders as EA's cannot be added to reparse points (which is what a FSA placeholder is). If you run in to this problem, then post here or ping me a private message. I believe we have some scripts you can use to remove the EA's.
Darren