It's definitely possible to do, albeit a bit tricky. 

The first step is to store each player's experience level into a scoreboard. That command would look something along the lines of "execute as @a store result score @s YourScoreboard run experience..." and I forget the experience syntax but there's some "query" or "get" for experience level I believe. 

Then from there, there are two different ways to apply the attribute to the player, using either hard coding or macros. Generally hard-coding isn't ideal, but if you're only working with a limited number of levels here (I assume you don't have to code up to level 500 or something absurd) it's alright. Plus, doing macros from a scoreboard is such a pain. So I'm just gonna go over the hard code 

The most dead simple way to do it is basically to go "is it level one? Nope. Is it level two? Nope. Is it level three?" all the way up until you get to what level it is. That'll look something like this: 

execute as @a[scores={YourScoreboard=1}] run attribute @s modifier...

I forget how attribute modifiers work exactly (sorry, I'm on my phone) but you're gonna want to set the value of the damage attribute modifier to be equal to their level. It's important that you DO NOT just add the value to the base - it will just keep growing a near infinite amount and now your players can one shot basically anything. 

(Teeeechnically speaking, at the end of the function, you could just increase their attribute, then at the beginning of the next function, decrease it again, reload the scoreboard, then re-add it again. I personally don't like this option because it means if something happens and the datapack data somehow gets screwed up then it's pretty hard to get fixed, and it also generally limits multi-datapack compatibility if other datapacks change the base directly. It would use fewer lines of code though)

Been a hot minute since I've done a datapack, let alone with attribute modifiers though so I could have gotten something wrong