Is it possible to install an external package in QGIS? For example: Can I install ReportLab in order to create a plugin to make pdf reports?
Or Pandas. How can I install Pandas in QGIS Python library?
QGIS has no ability to install Python plugin dependencies. You need to notify user to install dependencies by itself or merge dependency code into your plugin. There are limited number of Python modules shipped with QGIS under Linux/Windows (not sure with Mac OS X):