You should make a jar of your library in the origin project first. Then import the jar into the KS project’s Drivers folder as @Brandon_Hein mentioned. This is the way you should follow.
If your KS project depends on several external jar files, the following post describes how to resolve the dependencies and import the relevant jar files into the project’s Drivers folder.
As far as I know this approach is the only way to resolve complex dependencies to external jars.