mass export from archive manager to pst file

I hope you can assist :)

using archive manager 5.3

we have approx 31 million archived emails - 18 million have attachments

There are a few thousand email accounts - a lot are no longer associated with our Active directory (users that have long since left the company)

were moving away from archive manager to use barracuda 

what could you advise we use or do in this situation ? 

ive seen used the searchexporter tool and that can export to PST and barracuda can import PST

i believe i would have to do one at a time using searchexporter?