Something like this should work:

Code (Text):

1. on first join:
2.     set {customxp::%player%} to 0
3.     set {customlevel::%player%} to 0
4.  
5. on death:
6.     if attacker is a player
7.     add 10 to {customxp::%player%}
8.     if {customxp::%player%} is greater than 4999:
9.         set {customxp::%player%} to {customxp::%player%} - 5000
10.         add 1 to {customlevel::%player%}
11.         send "You leveled up to: %{customlevel::%player%}%" to attacker

This will give each individual player a customxp and custom lvl, then you just need to add to this whenever you want them to gain xp. (The example I gave is when the player kills an entity)

 

There is an "on experience change" function, which will imitate an experience orb collection, but you need to download the addon SharpSK for it. Also you can reference the variables across separate scripts if they are not local.