I made a feature for this plugin...
https://www.spigotmc.org/resources/mccombatlevel-expansion.47040/
It get the level of the player and returns it as a string to PlaceHolderAPI. Since PAPI is supported by a lot of plugins, I suggest you do it in your plugin.
Here is the source code for the expansion:
https://github.com/pixar02/McCombatLevel-Expansion

Thank you very much.

I suggest you do it in your plugin.

For this you could create a pull request or I could just create a reference to your expansion on the plugin page?

I think a reference should be fine. I don't think it has any advantages to make a pull request, and "hard code".