No, users are not able to apply Retention Policies directly to folders or messages. Archive Manager Administrators cannot create Retention Policies that are visible from the User level in the Web Interface for them to apply against the items in their Archive Manager mailbox by right-clicking on them and applying a set policy of their choice.
There is no way for a user to Right-click on a Folder or Message and apply specific policies from the Archive Manager Web Interface.
Currently in Archive Manager Administrators are able to create Retention Policies defined based on Message Tags, and users are able to Tag messages. If the Message Tag matches the defined Retention Policy criteria, the policy will be applied against the mail item.
Enhancement Request TF00716941 has been submitted to Development for consideration in a future release of Archive Manager.