You can set archiving at the channel level (so a folder channel) but by the sounds of it you are looking to archive individual folders within a channel. This can be done through the use of the Desktop Client.
(If you don't have the Desktop Client, it, and its documentation, can be found here: https://support.igloosoftware.com/desktop)
1) Create a new Folder Channel.
2) Use the Desktop Client to move the desired folder into the new channel.
3) Turn the archiving on in the new channel.
You can then either use that new Folder Channel to store archived material or you can move the folder back to its original location. However if you move the folder back you will have to wait a minimum of two days: the minimum archiving policy period is one day and the minimum grace period is also one day so it will take two days for the archiving to take effect (at minimum settings).
Let me know if you have any further questions,