I've added experimental support for this in the current development version (which will become v1.4.5 quite soon) https://wordpress.org/plugins/loco-translate/developers/
You have to enable the feature on the settings page, then if you have core translations available in your languages folder you should see a list of packages below themes and plugins on the main listing page.
You'll need at least one PO file or a POT file in each namespace to see that package appear. e.g.
This is experimental, because it has some difficulties in the way core translations differ from plugin and theme packages. Core translations are split across multiple PO files, although they share the same
default text domain. They also have overlapping source code directories, so it's not possible (using the plugin's current library code) to extract strings from PHP source.