Hi,
first add a field in Many2many fields model(table) then this field add in xml file as per your need.
for example:

tax_ids = fields.Many2many('account.tax', 'account_tax_default_rel', 'account_id', 'tax_id', string='Default Taxes')

* tax_ids model is " account.tax ", first you can add additional field in this model. then after add  in xml for "account.tax" views. like: tree,form etc.
* lots of example available in  odoo addons