build maven project with propriatery libraries included

1 Either you can include that jar in your classpath of application
2 you can install particular jar file in your maven reopos by

mvn install:install-file -Dfile=<path-to-file> -DgroupId=<group-id> \
    -DartifactId=<artifact-id> -Dversion=<version> -Dpackaging=<packaging>

Possible solutions is put your dependencies in src/main/resources then in your pom :

<dependency>
groupId ...
artifactId ...
version ...
<scope>system</scope>
<systemPath>${project.basedir}/src/main/resources/yourJar.jar</systemPath>
</dependency>

Note: system dependencies are not copied into resulted jar/war
(see How to include system dependencies in war built using maven)

Why not run something like Nexus, your own maven repo that you can upload 3rd party proprietary jar files, and also proxy other public repositories, to save on bandwith?

This also has some good reasons to run your own maven repository manager.

Create a repository folder under your project. Let's take

${project.basedir}/src/main/resources/repo

Then, install your custom jar to this repo:

mvn install:install-file -Dfile=[FILE_PATH] \
-DgroupId=[GROUP] -DartifactId=[ARTIFACT] -Dversion=[VERS] \ 
-Dpackaging=jar -DlocalRepositoryPath=[REPO_DIR]

Lastly, add the following repo and dependency definitions to the projects pom.xml:

<repositories>
    <repository>
        <id>project-repo</id>
        <url>file://${project.basedir}/src/main/resources/repo</url>
    </repository>
</repositories>

<dependencies>    
    <dependency>
        <groupId>[GROUP]</groupId>
        <artifactId>[ARTIFACT]</artifactId>
        <version>[VERS]</version>
    </dependency>
</dependencies>