Is it possible? Yes.
It just depends how you want to add it. By default, Lansweeper does not have an option to add license keys you already know. It's designed to scan them. However, you can add the keys to the database regardless if you are confident with SQL scripts.
You would have to check first how many license keys have been scanned so you can determine at which SerialID you can start. Then you can modify the report below and change the fields to have the correct values.
SET IDENTITY_INSERT tblserialnumber ON
insert into tblSerialnumber (SerialID, AssetID, Product, ProductID, ProductKey, Lastchanged)
VALUES
(1,1,'Microsoft Office 2016', 'ProductID','ProductKey', GetDate()),
(2,2,'Microsoft Office 2016', 'ProductID','ProductKey', GetDate()),
(3,3,'Microsoft Office 2016', 'ProductID','ProductKey', GetDate())
SET IDENTITY_INSERT tblserialnumber OFF
The alternative is to add it to an asset custom field either trough the web console or again, using a script.