It's possible to import a metaplugin back into FFRend as a project. This allows you to edit and re-export a metaplugin, even if you don't have the project file from which it was originally created. To import a metaplugin, use File/Metaplugin/Import. Select a filename, and press OK.
Note that if you only want to know which plugins a metaplugin uses, it's not necessary to import the metaplugin; it's easier to use the file browser. Find the metaplugin in the file browser's Plugins pane, right-click the metaplugin's name, and select Properties. The properties dialog lists the component plugins, and tells you whether they're linked or embedded.