Addin comes from VS we name COM and you can write it using C# or VB.NET
Addin build in VBA then XL file addin with VBA Code.
You can not transfer one to another.
If you like to use COM as Addin file you should rewrite it using VBA code (that id another lang do define and assigned objects).
However you have list of file Addins in XL Application too, but you can not use it by default. Usually you can run a code file that is an add-on
Solver that is a good example.