I believe there is no such call - I think you have to use an external app.
I hope somebody will come along and prove that to be wrong..
Frankly I was hoping for some "undocumented API" But there is no such a call - I'll go for external applications.
Unfortunately you will need to rely upon an external application for zipping and unzipping. My backup plugin has been out since LR2 and I spent considerable time looking for ways to zip or unzip using inbuilt functionality within Lightroom.
Info-zip is.the zip distribution used by most people who embed the functionality with their plugins. OS/X's native zip uses this version so include the Windows version with your plugin and it will simplify cross-OS compatibiltiy. Also their licencing is pretty flexible. Just keep in mind that if you want to use the test argument to verify your newly created zip isn't corrupt, you will need to distribute unzip with your plugin as well.