From the memo detail view you have the ability to Append (add items) to a memo.
Bring up the memo detail view for the memo you wish to append to. https://help.dcit.com/en/articles/2017041-how-do-i-view-the-memos-i-have-created-for-my-clients
Click the gear icon and choose the Append to this Memo option.
You will be teleported to the Append memo screen. From here click on the Add a new line option.
Search for the item in the field provided and select it. Set the qty and make any description and or price adjustments you may need.
Click the Save icon to save
You will now be transported back to your memo detail view with the changes.