Ok ! I've tested it ! It works fine, I've no problem.
In my opinion, jumping has a practical use in the game : it would be useful to jump over fences of a garden in order to quickly take cover in a house (instead of going round the house until you find the entrance while being under enemy fire...
).
Code (jump.sqs) :
_Nameofunit = _this select 0
; How high the person jumps, max 10, min 3, approx.
_howmuch = 5
; If the person jumps higher the more you jump
_jumpinc = 1
_wherex = getpos _Nameofunit select 0
_wherey = getpos _Nameofunit select 1
_wherez = getpos _Nameofunit select 2
_fastx = velocity _Nameofunit select 0
_fasty = velocity _Nameofunit select 1
_fastz = velocity _Nameofunit select 2
_newfastz = _fastz + 5
_Nameofunit setpos [_wherex,_wherey,_wherez]
_Nameofunit setvelocity [_fastx, _fasty, _newfastz]
; Stop the person jumping repeatadly, 3 should be enough of a delay
; It remove the jump acction, waits 3 seconds, and add's it again
_Nameofunit removeaction jumpacc
~3
jumpacc = _Nameofunit addAction ["Jump","jump.sqs"]
;and add to the player jumpacc = this addAction ["Jump","jump.sqs"]
exit
TOPIC SOLVED !