Giving external input to Triple Speed Ethernet

Yes it will work, as long as your custom GEN module generates a correct Ethernet Header. If you don't have the header, it will be dropped by the receiving end.

As for the line code from your previous message, it does a 5-bit rotation of the generated value on the left, and adds a constant value to the result.

The

(next_value << 5) & 32'hFFFFFFE0

part shifts the value 5 bits on the left, and the

(next_value >> 27) & 32'h0000001F)

part gets the 5 most significant bits (that are dropped by the previous shift) and put them back in the 5 low bits of the new value. Those two operations combines do the rotation.