tinyMCE.addEvent(tinyMCE.getInstanceById('<element>').getDoc(),"keydown",yourEventHandler);

/* Import plugin specific language pack */
tinyMCE.importPluginLanguagePack('banglafkb', 'en');
<script language="javascript" type="text/javascript" src="jscripts/test.js"></script>
var TinyMCE_KeywordPlugin = {
    getInfo : function() {
        return {
            longname  : 'Bangla Web based keyboard for tinyMCE',
            author    : 'Sabuj Kundu',
            authorurl : 'http://manchu.wordpress.com',
            infourl   : 'yet to add',
            version   : tinyMCE.majorVersion + "." + tinyMCE.minorVersion
        };
    },

    getControlHTML : function(cn) {
        switch (cn) {
            case "banglafkb":
                var html = '<select id="{$editor_id}_bnkeyboardSelect" name="{$editor_id}_bnkeyboardSelect" onfocus="tinyMCE.addSelectAccessibility(event, this, window);" onchange="tinyMCE.execInstanceCommand(\'{$editor_id}\',\'bnKeyboardSelect\',false,this.options[this.selectedIndex].value); this.selectedIndex=0;" class="mceSelectList"><option value="">{$lang_banglafkb_keyboard_default}</option>';
html +='<option value="bnenglish">{$lang_banglafkb_keyboard_english}</option>';
html +='<option value="bnphonetic">{$lang_banglafkb_keyboard_phonetic}</option>';
html +='<option value="bnunijoy">{$lang_banglafkb_keyboard_unijoy}</option>';
html +='<option value="bnprobhat">{$lang_banglafkb_keyboard_probhat}</option>';
html +='<option value="bninscript">{$lang_banglafkb_keybaord_inscript}</option>';
                html += '</select>';
                return html;
        }

        return "";
    },
    /*
    In Netscape 4 the keydown event fires only once, when the user initially depresses the key. 
    In all     other browsers there is no difference between keydown and keypress.
    */
    handleEvent : function(e) {    
            switch (e.type) {
                    case "keypress":                
                        return false; 
                    case "keydown":
                //test check for char 'v'    
                        if(e.keyCode=="86"){
                            tinyMCE.execCommand('mceInsertContent', false, '\u09AD'); //v
                        }
                                return false;               
                    case "keyup":                    
                            return false;
                }
        return true;
    },    
    execCommand : function(editor_id, element, command, user_interface, value) {
        // Handle commands
        switch (command) {
            case "bnKeyboardSelect":  
                switch(value){
                    case "bnenglish":
                        alert("English keyboard layout is activated");
                        return true;
                    case "bnphonetic":                        
                        alert("Phonetic keyboard layout is activated");
                        return true;
                    case "bnunijoy":
                        alert("Unijoy keyboard layout is activated");
                        return true;
                    case "bnprobhat":
                        alert("Probhat keyboard layout is activated");
                        return true;
                    case "bninscript":
                        alert("Inscript keyboard layout is activated");
                        return true;
                }//end inner switch
                
                //tinyMCE.execInstanceCommand (editor_id, 'mceInsertContent', false, value);
                return true;
        }

        // Pass to next handler in chainI am
        return false;
    },
    /*    
    function _parsePhonetic(){
            var e = (window.event) ? event.keyCode : evnt.which; // get the keycode
            alert(String.fromCharCode(e));
            return true;
    },
    */
    handleNodeChange : function(editor_id, node, undo_index, undo_levels, visual_aid, any_selection) {
    }
};

tinyMCE.addPlugin("banglafkb", TinyMCE_KeywordPlugin);