Currently mojoportal doesn't use transactions so no transactions have been included in the NpgsqlHelper. NpgsqlHelper was written by Joseph Hill for mojoportal, it is not part of the Npgsql library. When Joseph built NpgsqlHelper, he based it on the SqlHelper class in the MSSQL data layer of mojoportal. The MS SqlHelper class does have overloads with transaction support so if you look in there you may get an idea of how you can extend the NpgSqlHelper to support transactions. You would create similar methods in the NpgsqlHelper class but using the ado.net stuff for Npgsql. So if the Npgsql library supports transactions and I think it does, you could easily add support yourself in the NpgsqlHelper class.
mojoportal includes the source code for npgsql, but it is not the newest version. If the version in mojoportal doesn't support transactions you may want to check the newest version of npgsql http://pgfoundry.org/projects/npgsql
If you do implement those methods in NpgsqlHelper, please contribute them back to the project so that others can use it as well.
Hope that info helps.