For easy, try the second linked doc using the admin console.

Or
If the command has any problem(in fact,it works smoothly), you can manually  create  a repository directory in <geronimo-home>/repository.
suppose the jar is impl.jar, put it into a directory,like <geronimo-home>/repository/com/group/impl/impl.jar
And then add a dependency in you deployment plan,like
<dependencies>
            <dependency>
                <groupId>com.group</groupId>
                <artifactId>impl</artifactId>
                <type>jar</type>
            </dependency>
</dependencies>

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Best Regards,
Rodger.