EXAMPLE 1:
 if (old_inv_status == "pendingApproval") {
        //Copy the filters from objSearch into defaultFilters
        var defaultFilters = invoiceSearchObj.filters;
        log.debug("before adding filter defaultFilters", defaultFilters);
        var customFilters = {};
        customFilters = {
          name: "internalid",
          operator: "noneof",
          values: [old_inv_id.toString()],
          isor: false,
          isnot: false,
          leftparens: 0,
          rightparens: 0,
        };
        defaultFilters.push(customFilters);

        log.debug("defaultFilters after adding filter", defaultFilters);
        invoiceSearchObj.filters = defaultFilters;








EXAMPLE 2:

var mySearch = search.load({ id: '851' });
var defaultFilters = mySearch.filters;
var customFilters = {};
customFilters = {"name":"custrecord_customer","operator":"anyof","values":["64468"],"isor":false,"isnot":false,"leftparens":0,"rightparens":0};        
defaultFilters.push(customFilters);
mySearch.filters = defaultFilters;