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. About 90 lines down from the top of the file there is a section labeled <!-- The JDBC Driver used by uPortal --> and below it are the three properties which must be configured.
The database specific examples listed on 05 Database provide detailed instructions for this process.