Consu: this seems like a really bad idea, but maybe an easy way is to set the sharing permissions on each member folder to give all logged in users "can add" role.
I'm sorry but I just can't give to every logged user the "can add" role.
I need that a single user should be able to add files to every other users folder (in the Members/username folder) even if he has no administrative rights.
Perhaps, simply, I can't do it.
I'm sure it can be done. You'd have to write maybe an upgrade step that iterates over all member folders and grants "can add" to all logged in users on each member folder. When you add a new member you would also need code to create that member's home folder and grant "can add" to all logged in users.