Hi Keith,
After researching and consulting with senior member, manually adding management agent to Dirsync tool is not supported.
I also found a blog talking about setting up one forest with multiple tenants. In the TechNet blog, our engineer mentioned this is now supported. You either have to user the FIM Connector for Office 365 or you can now user multiple Dirsync servers syncing to each unique tenant. For detailed information, please click on the link below.
http://blogs.technet.com/b/educloud/archive/2013/08/02/multi-forest-and-multi-tenant-scenarios-with-office-365.aspx
However, this is not supported officially due to there is no official document announce about this feature.
If I misunderstand your requirement, please feel free to let me know. I will drive my best to help you.
Thanks,
Yang Yu
