You could edit the member_tooltip template. Better still, create a new template modification for this template. I think you'd want to make the edit before the userblurb tag, <xf:userblurb user="$user" tag="div" />.
Thanks for replying...I am not a proficient programmer...
Could you give an example of adding a custom field, also how do you refer to the field name of the custom field that has been added...
I think you can use: {$user.Profile.custom_fields.fieldId} (where fieldId is your custom field Id) to show a user's entered value for a particular field.