Hi,
Yes you can manually add sales from other platforms, you just need to make sure you have all of the required fields so that the data can be pulled into the various reports.
The required fields are:
"Item/Transaction ID (Unique)" - this can be filled in with any unique value. Note: this column is hidden from view by default.
"Quantity" - self explanatory
"Sale Date" - self explanatory
"Month Sold" - First day of the month of Sale date, can be populated by formula if you choose. Note: this column is hidden from view by default.
"Sale Price" - self explanatory