Yes, all Sealevel relay racks allow an external power source to be connected to the V+ and Ground terminals. However, you need to take steps to prevent the external power source from back-feeding current into the Sealevel TTL I/O adapter connected to the relay rack.
On single point discrete relay racks, you need to clip the wire labeled ‘W1’ before connecting external power. On QSSR relay racks, you need to clip the resistor (or remove the fuse) connected to pin 49 before connecting external power. This isolates the external power from the Sealevel I/O logic levels, but maintains the Ground connections.
In certain applications, using an external power source allows Sealevel TTL I/O adapters and relay racks to be used with a wider range of relay modules that have power requirements other than the +5V supplied by Sealevel TTL I/O adapters.