Does SQLDeveloper have a Toad-like tool to copy/clone an Oracle database user?
If not, I'll resort to: How to Clone a User in Oracle
In version 3 there is a DBA view. It can be displayed or hidden using the view menu. Once you have it displayed open the connection in the DBA section. Drill down to Security->Users and right click on the user you want to copy. The option you are looking for is create like.