uPortal uses Maven for dependency management. To add your JDBC driver you need to know the Maven groupId, artifactId and version for the JAR and add this information to the /uPortal/pom.xml file. The information is configured in the <properties> block about 90 lines down in a section that looks like:
<!-- The JDBC Driver used by uPortal -->
<jdbc.groupId>hsqldb</jdbc.groupId>
<jdbc.artifactId>hsqldb</jdbc.artifactId>
<jdbc.version>1.8.0.7</jdbc.version>
The database specific examples listed on 05 Database provide detailed instructions for this process.