I can, but the event system is ill-suited for what you are trying to achieve. You will have to make n conditional branches for n levels.
variables:
FishingLevel
FishingExp
Fishing Event:
FishingExp +50
If FishingLevel <2
If FishingExp => 300
Add FishingLevel +1
Show text: You gained a fishing level. Your fishing level is now \pv {fishinglevel}
ELSE
If FishingLevel <3
IfFishingExp=> 600
Add FishingLevel +1
etc etc etc etc.
This should work. Haven't got the editor open to test.
However, as you can see. You need a stupid amount of conditional branches, so I would suggest to keep the levels to something like: Beginner, Decent, Expert, Master (Or something like that).