Wow, lol
I know, hell you would really howl if you saw me trying to solder one on after the fact.
either include it with the existing SKU or add a new one. My money is on the add a new one because they(seeed) isn’t giving anything away for free… (unless it’s a live stream!) 
HTH
GL 
PJ
while i’m soap boxing it…
Why not put it on the Xiao ESP32S3 only sku# , why? Well otherwise it’s not expandable or taking advantage of even having a new MCU. It would basically be the same as a “LAME” ESP32C3. IO wise with out it. C’mon seeed DO the RIGHT thing.
Please. 
and the sales page says,
Interface:
1x UART, 1x IIC, 1x IIS, 1x SPI,11x GPIO(PWM), 9x ADC, 1x User LED, 1x Charge LED
they should qualify the statement to the real world use. You can’t use 11 or 9 there are not enough pins.
so really to be true and correct the connector should be installed. IMHO 