Evgeni,
Currently there is no way of packaging an extension directly from the command line, also regarding the ./mage package
command
Connect commands available:
===========================
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package Build Package
package-dependencies Show package dependencies
package-prepare Show installation information of package
......
The command names can be misleading the package in particular is designed to be ran on the package.xml which contains a module information, it basically has the opposite effect of what you are trying achieve.
Currently I don't think there is any solution to package extensions directly from the command line, I supposed that you could create a script that does this but it would be hard to specify all the extension files, dependencies and information that is required to generated a valid Connect extension package.