The current Xcode plugin only supports building .app and .ipa files. It would be great to add an option to build Xcode archives (.xcarchive) files as well.

This can be done from xcodebuild, for example:
xcodebuild -target "My Target" -scheme "My Scheme" -configuration Release clean archive

An alternative would be to have a post build step that can take the output .app and .dSYM files and bundle them up as a .xcarchive.