Don't remember specifically for Chroma Key, but most plug-ins have numerical values that you can copy on a piece of paper and type in on the new computer. It's a pain, but it works.
Custom presets get saved in the project file. Save your custom settings as a preset and save a .vf file with those settings applied somewhere. Load that .vf file on the other computer. Open up the .vf file and then open up the plugin on the other computer. Save it as a custom setting on that computer too.