I use generic setup to setup the memberdata properties. Two Files are available, but i'm not sure if it necessary that both are manipulate. memberdata.xml and memberdata_properties.xml. you can export these files via portal_setup.
portal_memberdata = getToolByName(site, "portal_memberdata")
if not portal_memberdata.hasProperty("home_folder_uid"):
portal_memberdata.manage_addProperty(id="home_folder_uid", value="", type="string")
Can you explain what you want to add, there are a few things to consider.
It is possible to 'just add a property
It is possible to let the user edit this (his/her) property
About edit: it is possible to let the user add it when register, or just when editing profile or both, or you can have a setting which only an admin can edit.
Some of this can be done TTW (in the browser), but others require 'some kind of programming'.
It is possible to add properties without an add on, try: yoursite/@@member-fields
If you have your own add on, it is easy to add with plonecli (ask here and I will answer)):
in /profiles/default you add a file memberdata_properties.xml (or memberdata.xml, I do not remember which is which).
In this file you add your property.
When you install your add-on, this property will be available.
The easies way to know 'how this file should look' is to go to yoursite//portal_setup/manage_exportSteps
and export the settings.
Then you 'just add your field to that memberdata.xml (or memeberdata_properties.xml)
Remember: you need to install the add-on to 'get the setting'