get object value in alarm.c

The  function below in Rmon/alarm.c intends to get a value of an object to
monitoring it, but only objects in internals mibs  can be gotten.
How can I get object of externals mibs that have the handler
_handler(netsnmp_mib_handler *handler, netsnmp_handler_registration
*reginfo, netsnmp_agent_request_info *reqinfo, netsnmp_request_info
*requests)?

static int
fetch_var_val(oid * name, size_t namelen, u_long * new_value)
{
    netsnmp_subtree *tree_ptr;
    size_t          var_len;
    WriteMethod    *write_method;
    struct variable called_var;
    register struct variable *s_var_ptr = NULL;
    register u_char *access;


    tree_ptr = netsnmp_subtree_find(name, namelen, NULL, "");
    if (!tree_ptr) {
        ag_trace("tree_ptr is NULL");
        return SNMP_ERR_NOSUCHNAME;
    }


    memcpy(called_var.name, tree_ptr->name_a,
           tree_ptr->namelen * sizeof(oid));

    if (tree_ptr->reginfo &&
        tree_ptr->reginfo->handler &&
        tree_ptr->reginfo->handler->next &&
        tree_ptr->reginfo->handler->next->myvoid) {
        s_var_ptr = (struct variable
*)tree_ptr->reginfo->handler->next->myvoid;
    }

    if (s_var_ptr) {
        if (s_var_ptr->namelen) {
                called_var.namelen =
                                   tree_ptr->namelen;
                called_var.type = s_var_ptr->type;
                called_var.magic = s_var_ptr->magic;
                called_var.acl = s_var_ptr->acl;
                called_var.findVar = s_var_ptr->findVar;
                access =
                    (*(s_var_ptr->findVar)) (&called_var, name, &namelen,
                                             1, &var_len, &write_method);

                if (access
                    && snmp_oid_compare(name, namelen, tree_ptr->end_a,
                                        tree_ptr->end_len) > 0) {
                    memcpy(name, tree_ptr->end_a, tree_ptr->end_len);
                    access = 0;
                    ag_trace("access := 0");
                }

                if (access) {

                    /*
                     * check 'var_len' ?
                     */

                    /*
                     * check type
                     */
                    switch (called_var.type) {
                    case ASN_INTEGER:
                    case ASN_COUNTER:
                    case ASN_TIMETICKS:
                    case ASN_GAUGE:
                    case ASN_COUNTER64:
                        break;
                    default:
                        ag_trace("invalid type: %d",
                                 (int) called_var.type);
                        return SNMP_ERR_GENERR;
                    }
                    *new_value = *(u_long *) access;
                    return SNMP_ERR_NOERROR;
                }
            }
        }

    return SNMP_ERR_NOSUCHNAME;
}