If all you are looking for is that the user types in a value = to a value in your list... 
Use a custom validator with this as your validation function:

function ValidateCombo(oCombo,ColIndex) {
    try {  

        var grid=oCombo.getGrid();
        var rowCount=grid.Rows.length;
        var currVal='';
        var comboText=oCombo.displayValue;

        for(i=0;i<rowCount;i++){
          var row=grid.Rows.getRow(i);
          currVal=row.getCell(ColIndex).getValue();
          if (currVal == comboText) {
            return true;
          }   
        }
        return false;
    }   
    catch ( err ) {
        var txtMessage = document.title + " - Error in ValidateCombo()\n" + err.description ;
        alert( txtMessage ) ;
    }  

}