Hi!
You are looking for the Advanced content field on the Advanced Options -> Content options panel: https://i.imgur.com/UOegjef.png
This documentation part will clarify almost everything about it’s usage: Advanced title and Description fields
Long story short, the product SKU is stored within the _sku custom field, and the advanced description field allows displaying custom fields content as well. In your case it would be something like this:
[html]{descriptionfield}<p>{_sku}</p>[/html]
..or if you want the SKU text to be bold:
[html]{descriptionfield}<p><strong>{_sku}</strong></p>[/html]
.. if the SKU field is not set on every product, then using the conditional brackets will make sure that the paragraph is only displayed is the _sku is set:
[html]{descriptionfield}[<p><strong>{_sku}</strong></p>][/html]
I hope this helps!