Did you try something like this:
<dependency>
<groupId>com.foo</groupId>
<artifactId>footron</artifactId>
<version>4.2</version>
<scope>runtime</scope>
<type>dll</type>
</dependency>
You can add them to maven’s repository with something like this:
mvn install:install-file -Dfile=footron.dll -DgroupId=com.foo -DartifactId=footron -Dversion=4.2 -Dpackaging=dll -DgeneratePom=true
Haven’t done this for DLLs but something like this should work.