function specialSuccessStuff(transport)
{
    // ...whatever your special onSuccess stuff is...
}

function makeRequest(url, options) {
    var orig;

    orig = options.onSuccess;
    if (orig) {
        options.onSuccess = function(transport) {
            specialSuccessStuff(transport);
            orig(transport);
        };
    } else {
        options.onSuccess = specialSuccessStuff;
    }
    return new Ajax.Request(url, options);
}