Solved in another way... Thanks to Mr.Mister_e and Mr.Darrel !!!
The hardware is now : col_0 = portB.0 ; col_1 = portB.1 ; row_1-3 = portA.0-2
Code:
@ DEVICE pic16F628A, INTRC_OSC, WDT_OFF, PWRT_OFF, BOD_OFF, MCLR_ON, LVP_OFF, CPD_OFF, PROTECT_OFF 

   
   Include "modedefs.bas"   ' Serial Protocol
   Define   OSC 4           ' 4MHz crystal used
   CMCON = 7                ' Disable on-chip comparator, PORTA in digital mode
              
   '// Define port pins as inputs and outputs ...
   TRISA  = %00000111
   TRISB  = %00000000

Buzzer    VAR PORTB.5

include "c:\pbp\keypad2.pbp" ' see http://www.picbasic.co.uk/forum/showthread.php?t=3250

main:
gosub keypadscan
gosub check
goto main

check:
select case key 
case 1
   high buzzer
   pause 1000
   low buzzer

case 2
   high buzzer
   pause 2000
   low buzzer
   
case 3
   high buzzer
   pause 3000
   low buzzer
   
case 4
   high buzzer
   pause 4000
   low buzzer
   
case 5
   high buzzer
   pause 5000
   low buzzer
   
case 6
   high buzzer
   pause 6000
   low buzzer
   
end select
Return

end 'of story !
Maybe it's useful for somebody !