how to create a multiselect combobox in extjs 3.2.1?

Question

I have a panel and inside it a combo, I have tried multiple ways, even with multiselected: true (but it seems to be for versions higher than 4), how could I do it in this old version?

{
xtype : 'combo',
id : 'cmbSociedad',
allowBlank : true,
forceSelection : true,
autoLoad : true,
blankText : campoNoVacio,
fieldLabel : contratoBuscadorSociedadKonecta,
emptyText : contratoBuscadorBlankTextSociedadKonecta,
store : storeSociedades,
displayField : 'nombreCif',
valueField : 'id',
selectOnFocus : true,
mode : 'local',
typeAhead : false,
editable : true,
triggerAction : 'all',
width : 500,
value : ''
}