Create a custom sequence with 320 x 320 resolution and import your footage with 320 x 240 dimensions. Make sure that scale to frame size is off. That will give you the top and bottom black borders.
How do I create a custom sequence? The only options I get in new sequence window are:
Mobile & Devices
Hi Clam 000,
If you are still unsure what to do, read the manual, get a proper education and consult the help file.
"I reserve the right to call anyone a noob. Especially if you haven't read the f****** manual!!"