Thanks dandfra, I've already find the solution,I use an override in this function:
Code:
Ext.override(Ext.ux.netbox.core.FilterModel,{
getFilterObj:...
Now, my main problem is about using the DWR with your filter plugin, and I need the mapping property in the fields.
Maybe i must override this function?
Code:
Ext.override(Ext.ux.netbox.core.FieldManager, {
createFieldFromCfg: function(fieldCfg){
and passing the parameters to these functions 
Code:
Ext.ux.netbox.string.StringField();
Ext.ux.netbox.string.EnumField();
Ext.ux.netbox.number.NumberField();
Ext.ux.netbox.date.DateField();
My goal is to use the same field configuration for grid and filter plugin :-)
Thanks again
S.