I am trying to export data from SourceOne archives of users who have left the organization to PST. I would like to be able to query all such users and export all their data out of the system (it's ok if this has to be done one by one). These users no longer have AD/Exchange entries anymore.
Is there a way to isolate this information either through Discovery Manager or a query directly against the database?
There is a section within SourceOne Email Management Administration guide called "Best practices in email identity management". This section would help you to plan for long run.
On next page you should find "Managing historical address information" section that talks about legacy employees. It also provide instructions on how to add back legacy addresses to run searches.
In short you need to identity users LegacyExchangeDN in exchange environment and the SMTP addresses. Once messages hit exchange servers internally they are referenced by LegacyExchangeDN mostly which is why that is utmost important part. In case you do not have record of this info, within SourceOne Archive Database (default name Es1Archive) there is EmailAddress table and you can run SELECT queries against it to look for this information.
Once you have addresses available you can either create a user in AD with all the addresses info or you can create an identity within Discovery Manager (Full edition, not express edition) with the addresses and run search against identity and then export results to PST.
If you need further help, I would encourage you to contact DELL EMC SourceOne support team by opening service request or chat request by going to support.emc.com website.
You do not necessarily have to create a record in your mail environment if the users no longer exist, however you can perform searches in Discovery Manager using the Discovery Manager client (Web or thick client) against identity records.
You can create identity records for each of the users you would like to run a search for in Discovery Manager and search against those identify records. Each identity record should include all the email addresses of the user including their canonical names prior to leaving the company.
See the Discovery Manager User guide located at https://support.emc.com/products/424_SourceOne-Discovery-Manager/Documentation/
The DM Collect Utility also allows you to create multiple searches at once. You will need to create custodians for each of the identify records in the matter then use the utility to run the search against all custodians.
I hope this is helpful.