The easiest way is by adding a module file to your code. When a variable is declared as public in a module, it is visible in the whole project. Having very public stuff is not a good habit, but for swApp, which always has the same value, I would say it is not that bad.
Public swApp as Sldworks.SldWorks
Then, in your addin class, just assign using Global.swApp = TheSWAppVariableYouHaveInYourClass