Import po-files using Drush
I found a way to do this using Drush and Localization Update.


Install Localization Update  drush en l10n_update
Configure a local path for po-files (admin/config/regional/language/update)
Configure automatic updates from remote servers or just local sources 


Now you can put your own po-files in a local path (for example sites/all/translations). If you have translations for your own module then you name it according to this naming convention:

modulename-version.language.po

So a Swedish po-file for 'my_module', version 7.x-1.1 would be named:

my_module-7.x-1.1.sv.po

Now you have everything setup and you can start updating your sites translation with Drush:

drush l10n-update-refresh # Looks for new po-files
drush l10n-update # Updates translations