Yes, you can.
The way the old Sunn speaker jacks are wired, the 16 ohm jack is connected to the 16 ohm tap of the transformer. The 8 ohm jack is connected to the 8 ohm tap, and is a switched jack. When you plug into the 8 ohm jack, the 16 ohm jack is disconnected from the 16 ohm tap and connected in parallel with the 8 ohm jack, so both jacks are then connected, in parallel, to the 8 ohm tap.
In short, there's not a thing wrong with plugging directly into the 8 ohm Extension Speaker jack.