Solution
Purchasing (replacement) sows:
Go to [1. Entry/Modify] - [1.1. Sows] - [1.1.1. Intake/Disposal]
Click 'New' to add an animal.
Enter the new sow's details; the green fields are mandatory.
Click the green checkmark to save the purchase.
Note! These animals are immediately put into production and count as sows within the program!