how to scan a whole sim with llSensor?

llSensor is limited to 96 meter, but is there a way to extend it to a whole sim?

Nope, limited to 96 meters really does mean limited to 96 meters. There are some bugs where it can return past that point but it isn't reliable. Whole sim scanners use one or more objects that move around and report back their findings to a central location. Scripting such a system isn't for the faint of heart or new scripters. And even if it is scripted right, it can still both contribute to and be affected by lag.

how can i store the sensor data until i want to retrieve the data?

The normal way would be to put it into a list. The structure of that list depends on what data you are going to need. For example, if you just want to know the UUID's of the avatars who were detected, you'd have something like this for your sensor event:

sensor( integer total_number) {
integer i;
for (i = 0; i < total_number; i++) {
 sensordata += [llDetectedKey(i)];
}
}

You would declare "sensordata" as a list near the top of the file, and you could easily change the above to code to store other stuff too in a 'strided' list.

so, if i put that stuff into my script i could store any data in it, not only sensor data...?

Yes, lists are quite versatile... you can store pretty much anything you like in them (except other lists):

http://www.lslwiki.net/lslwiki/wakka.php?wakka=list

You can also create as many lists as you need for different purposes (within the memory limitations, obviously!).