The only way to do this is to drop your jar files into the lib/core/gateway, lib/core/client, lib/core/designer, or lib/core/common folders, depending on the scope you want to be able to access them in. Putting them in common should make them available everywhere. The downside is that these are not part of the backup or preserved in any way, so you need to make sure your jars are always added to those locations in new gateways, etc… on your own.
disclaimer: this is not officially supported and there are no guarantees this will continue working in the future.
If you go the module route you can add your own scripting functions but the classes your module uses won’t be in the right class loader to use the way you’re asking.