class Qualification(models.Model):
    _name = "hr.qualification"
    _order = "name"

    name = fields.Char(string="Qualifikation")
    date_begin = fields.Date(string="Datum Erwerb")
    date_duedate = fields.Date(string="Datum Ablauf")
    qual_type = fields.Many2one("hr.qualification_category")

Now in Search Box in Kanban view I can a custom filter on qualification by Filters->Add Custom Filter, 
which in fact searches hr.qualification.name. However, is there any way to have fields date_begin resp. date_duedate
available as searcheable field in Custom Filter?

Thanks and regards!
Matthias