Just for logging: HP N54L could be upgraded to DSM 6.2.2-24922 with JUN'S LOADER v1.03b - DS3615xs
As several times mentioned above, you need to add an additional NIC, as the internal NIC will not work after DSM 6.2-23739 Update 2.
I added a 2 port HP NC360T and deactivated the internal NIC in the BIOS. Update ran without problems directly via DSM.