I've been googling and I've found that I can remove/add class (e.g, required, number); that gave me some ideas on how to simulate validation groups.
$(document).ready(function(){     
    $("#add").click(function() {
        $("#fecha").removeClass("required");
        $("#fecha").removeClass("date");
        $("#numeroDoc").removeClass("required");
        $("#numeroDoc").removeClass("number");
        $("#serieDoc").removeClass("required");
        $("#serieDoc").removeClass("number");
        $("#importe").addClass("required");
        $("#importe").addClass("number");
        $("#concepto").addClass("required");           
    });
    $("#save").click(function() {
        $("#importe").removeClass("required");
        $("#importe").removeClass("number");
        $("#concepto").removeClass("required");    
        $("#fecha").addClass("required");
        $("#fecha").addClass("date");
        $("#numeroDoc").addClass("required");
        $("#numeroDoc").addClass("number");
        $("#serieDoc").addClass("required");
        $("#serieDoc").addClass("number");  
    });
    $("#form1").validate({           
        errorPlacement: function(error, element) {
            error.appendTo( element.parent("td").next("td") );
        } ,
        submitHandler: function(form) {              
            $("#add").click(function() {
                addDetail();
                return false;
            });
            $("#save").click(function() {
                return false;
            });                      
        }
    });
});
Though, I've apparently found the answer, I'm not so sure about it due to some details:
1.When the page first loads, the numbers only validation (class="number") doesn't work till I press Add
2.If I make a mistake by entering the data then I have to press twice the button to fire the validation
I hope you can help me improve this code