I have seen the same issue with 64 bit drivers and HF4
A quick workaround was to copy an older version of DriverManager64.exe to a separate folder on the server - you can open the original symantec_deploymentsolution_7_5_x64.msi found in the installation folder \Program Files\Altiris\Symantec Installation Manager\Installs\Altiris\ in 7zip to find it.
Run it, and Click the change button to direct the DriverManager to a temporary folder.
Click the Add Driver option, browse and import the drivers.
In the temporary directory you will now find directories for each of the selected drivers, copy these directories to the Program Files\Altiris\Deployment\DriversDB directory, and delete the drivers.manifest.txt file within it.
Then in the console go to Settings / Deployment / Driver Management, the drivers.manifest.txt will be re-created, including your additional drivers.
Phil