How to create lots of SQL-Developer Connections quickly?
SQL Developer stores its connection information in (on Windows):

%HOMEPATH%\Appdata\Roaming\SQL Developer\system18.1.0.095.1630\o.jdeveloper.db.connection\connections.xml


The path obviously changes within different versions because of the version number.

I am not aware of any built-in solution for adding entries except the GUI, but it is not that difficult to copy an existing entry and modify the values. Passwords are stored in a hashed form, copying an existing hash value works as well.