Moving retired clients into their own collection/group

I currently have the clients deleted out of the database after 60 days of
inactivity. Even though these machines are no longer on our network, I need
to save the historical data. Is there a (relatively) easy way to move
retired clients out of 'All Systems' and into their own 'Retired Systems'
collection, etc?

If you are deleting the aged data, then unfortunately you won't be able to
have a collection for Retired Systems. However, if you turned off the
deletion of the aged data, then you could create what the Retired Systems as
the All Systems collection is just a collection with a query that can be
modified.

Otherwise you may want to look into a tool for a CMDB.