Update: my comment below this answer was intended for another post please disregard it. This answer should work just fine!
Example: Create a relationship between product.product to a new model named virtual_categories.

Hope this helps!

from openerp import fields
from openerp import models
class virtual_categories( models.Model ):    
    _name = "virtual.categories"    
    name = fields.Char( string = 'Virtual Category Name' )    
    virtual_category_id = fields.Many2one( 'product.product', string = 'Virtual Category' )