A walk-in visit will normally be processed by a shop that does not require patient profile. Meaning you will operate like a grocery store without patient history.
To do that from the menu:
2-Start a walk-in visit
You will then be presented with the shopping cart.
You may still add a patient once you started a walk-in visit. To do so:
From the shopping cart:
1-Select add patient
2-Search for the patient you want to add