Starting POS customization requires understanding that point of sale app is a offline capable application.

This code will add an existing field to the offline model of the PoS

var models = require('point_of_sale.models')
var _super_posmodel = models.PosModel.prototype;

models.PosModel = models.PosModel.extend({
initialize: function (session, attributes) {
        // Add field to model
        var partner_model = _.find(this.models, function(model){
            return model.model === 'res.partner';
        });
        partner_model.fields.push('membership_code');
        // run Super
        return _super_posmodel.initialize.call(this, session, attributes)
        },
});