If you have already downloaded a plugin's .nbm file, you can manually install it without having to connect to an update center.
To install a plugin offline:
Choose Tools > Plugins from the main menu and select the Downloaded tab.
Click Add Plugins and browse to the location of the downloaded .nbm file. Select the file and click Open.
Repeat the previous step for each plugin that you wish to add.
Click Install and complete the pages of the installer to install the plugin.
If the plugin that you have selected in the file chooser does not appear in the Downloaded tab of the Plugin Manager, you probably have the same or a newer version of that plugin already installed. You can check the Installed tab to verify that the plugin is installed and activated.