You can't set an altitude for a person me thinks. but if that person is on a bike then you can set the altitude on the bike and thus for the person riding it as well.
btw the car will drop like a stone. If you need to keep it up you have to make a script with a loop.
For example:
create a car and name it car1
create a carfly.sqs and put in the cars init field
[
timetohover,velocity] exec "carfly.sqs"
put this in the carfly.sqs
_count = 0
_timeline = this select 0
_vel = this select 1
#start
_count = _count +1
car1 setpos [getpos car1 select 0, getpos car1 select 1,(getpos car1 select 2) +10]
_height = getpos car1 select 2
car1 setvelocity [0,0,_vel]
~0.01
?_count == _timeline: _vel = 5
?_height < 1: exit
goto "start"
timetohover is the amount of times the script will loop itself. thus timeline 10 would keep the car in the air for 0.1 seconds. timeline 100 for 1 second and so on.
velocity is the velocity the car is pulled from ground. if you increase the velocity then the car will go upwards - decrease it and the car will lower. Try first velocity of 50 and see what it does.
I just wrote this at school so I have no idea if it has bugs or anything, but give it a go and fiddle with the values. It should work.