I found by myself how to do that :
First, I needed to define the following variable to send the numpad keys and not the ones which are above the QWERTY keys :
#define KEYPAD_1 ( 89 | 0xF000 )
#define KEYPAD_2 ( 90 | 0xF000 )
#define KEYPAD_3 ( 91 | 0xF000 )
#define KEYPAD_4 ( 92 | 0xF000 )
#define KEYPAD_5 ( 93 | 0xF000 )
#define KEYPAD_6 ( 94 | 0xF000 )
#define KEYPAD_7 ( 95 | 0xF000 )
#define KEYPAD_8 ( 96 | 0xF000 )
#define KEYPAD_9 ( 97 | 0xF000 )
#define KEYPAD_0 ( 98 | 0xF000 )
Then I used this code :
DigiKeyboard.update();
DigiKeyboard.sendKeyPress(KEYPAD_0,MOD_ALT_LEFT);
DigiKeyboard.sendKeyPress(KEYPAD_1,MOD_ALT_LEFT);
DigiKeyboard.sendKeyPress(KEYPAD_8,MOD_ALT_LEFT);
DigiKeyboard.sendKeyPress(KEYPAD_3,MOD_ALT_LEFT);
DigiKeyboard.sendKeyPress(0,0);
DigiKeyboard.delay(100);
The "trick" was to use "sendKeyPress()" instead of "sendKeyStroke()" :
- sendKeyPress(keyid,mod) : press the keys "keyid" with the modifier "mod" but release none of them
- sendKeyStroke(keyid,mod) : press the keys "keyid" with the modifier "mod" and release all of them
BUT : in order to keep ALT pressed during the whole operation, it is important to use it as a modifier in each line
At the end, you can use sendKeyPress(0,0) to release all the keys.
I hope that it will help some people in the future

G