Assign a Folder # to a user
I'm assuming that you have already imported all your members into Joomla and Civicrm.

It seems to me that you will need at least 2 new custom fields named say 'folder number' and 'instrument name' into which you will make manual entries, and a new dataset (perhaps called 'Membership Information'). The newly created custom fields would then need to be added to either a new or existing profile.

You would be advised to read the custom fields documentation at https://docs.civicrm.org/user/en/stable/organising-your-data/creating-custom-fields/ and profiles at https://docs.civicrm.org/user/en/stable/organising-your-data/profiles/

Hope this helps.