Yes, it is possible, for migration purposes, what is stopping you from setting up the connection with 18.x "now"?
. Download the latest jtds ODBC driver:
. Unzip and then in sqldeveloper under "Tools -> Preferences... -> Database -> Third Party JDBC Drivers -> Add Entry...
. Make sure NTLMAUTH.dll is accessible via PATH.
. Set up sql server connections under the "Connections -> SQL Server" tab that is now there:
I've done it somewhat recently, so yes.
What happens when you try? And also, what VERSION of SQL Server are you trying to migrate to Oracle?