ExtraPathDirectories parameter can be used to add extra test libraries to your build.
By default, you should add them to ${project.basedir}/src/test/resources/robotframework/libraries.
This is just and example how importing can be done, as there's also Java-based Selenium-library available that can be included just as Maven depednency
Download the tarball from the Selenium Library releases page, e.g. robotframework-seleniumlibrary-3.3.1.tar.gz
Extract the file and move the SeleniumLibrary directory from robotframework-seleniumLibrary/src to $project.basedir/src/test/resources/robotframework/libraries.
So your project will have a folder ${project.basedir}/src/test/resources/robotframework/libraries/SeleniumLibrary
Make same thing for external dependencies of the library. Robot Framework doesn't need to be imported, as it comes from dependencies.
Now, you can import SeleniumLibrary and use its keywords in your tests.
Below you see a test case that uses the Open Browser keyword. The Selenium server is started during suite setup.
*** Settings *** Library SeleniumLibrary Suite Set Up Start Selenium Server Suite Tear Down Stop Selenium Server *** Test Cases *** My Test Open Browser http://robotframework.org