Hi,
You can manually add sales from other platforms; the key is to make sure to input all of the required fields.
One of these that isnt obvious is the "Item/Transaction ID (Unique)" column (Column B) which is hidden by default. You can fill this in with any unique value (eg. Posh000001, Posh000002, etc)
The other required columns are a bit more obvious, (like sales date, price, etc) but if you still have trouble please let me know.