I'm aware I can add maven repositories for fetching dependencies in ~/.m2/settings.xml. But is it possible to add a repository using command line, something like:
mvn install -Dmaven.repository=http://example.com/maven2
You can do this but you're probably better off doing it in the POM as others have said.
On the command line you can specify a property for the local repository, and another repository for the remote repositories. The remote repository will have all default settings though
The example below specifies two remote repositories and a custom local repository.
mvn package -Dmaven.repo.remote=http://www.ibiblio.org/maven/,http://myrepo -Dmaven.repo.local="c:\test\repo"