I think you have two options ..
1)
If you have already processed these original PSTs and have new ones output already, then you can use either server side or client side migration from Enterprise Vault, or use the VAC to import them.
2)
If you intend to process the original PSTs item by item and create new PSTs then as you process each individual item in the source PST, you could use the ECM API to pump the message in to EV at the same time as you drop the new message in to the new PST.
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