I've made an extension to the SendKeyStroke method of DigiKeyboard.h by adding a key down time parameter.

SendKeyStroke method now has a third parameter to allow the simulation of pressing time (in milliseconds) that the key remains pressed until is released.

void sendKeyStroke(byte keyStroke, byte modifiers, uint16_t key_down_time);

If 'key_down_time' is zero then don't release the key, it should be done manually by sendKeysStroke(0). As 'zero' means forever, the minimum press time time is 1ms and the maximum is 65535ms.

Examples:

DigiKeyboard.sendKeyStroke(KEY_B);        // Send a B to host
DigiKeyboard.sendKeyStroke(KEY_A,0,300); // Send an A to host, takes 300ms until release

DigiKeyboard.sendKeyStroke(KEY_C,0,0);    //Send a C to host and keep it pushed
DigiKeyboard.delay(2000);                 // wait 2 seconds
DigiKeyboard.sendKeyStroke(0);            // now release any key pressed.

Tried to send a pull request to original project but without success, though. Either way the extended function is below:

  void sendKeyStroke(byte keyStroke, byte modifiers, uint16_t key_down_time) {
   	while (!usbInterruptIsReady()) {
      // Note: We wait until we can send keystroke
      //       so we know the previous keystroke was
      //       sent.
    	usbPoll();
    	_delay_ms(5);
    }
    
    memset(reportBuffer, 0, sizeof(reportBuffer));
		
    reportBuffer[0] = modifiers;
    reportBuffer[1] = keyStroke;
    
    usbSetInterrupt(reportBuffer, sizeof(reportBuffer));
		
  	while (!usbInterruptIsReady()) {
      // Note: We wait until we can send keystroke
      //       so we know the previous keystroke was
      //       sent.
    	usbPoll();
    	_delay_ms(5);
    }
	
	// Pressing time (in milliseconds) that the key remains pressed
	// until is released. If is zero don't release the key (it should be
	// done manually by sendKeysStroke(0). Therefore minimum time is 1ms
	// and the maximum is 65535ms.
	if (key_down_time) {
		while (key_down_time--) {
			usbPoll();      
			_delay_ms(1);
		}
		// Now send 'no key' pressed
		memset(reportBuffer, 0, sizeof(reportBuffer));
		usbSetInterrupt(reportBuffer, sizeof(reportBuffer));	
	} 
  }