Step 1: Take a full backup of your database. Assume that it will be unrecoverable once you touch DUAL.
Step 2: Issue the DDL or DML statement. You'll probably need to be logged in as SYSDBA in order to properly corrupt your database.
Step 2a: Watch all the weird and wonderful errors that your code and Oracle starts to spew.
Step 3: Shut down the database and recover from backup.
Changing any table in an Oracle-supplied schema, such as SYS, is not a real good idea.
If you're thinking "It would be great if dual had 2 rows" (or 5 columns, or both), "then I could do ...", then create your own table, in your own schema (use CREATE TABLE ... AS, if that helps), and use your own version rather than the one that Oracle provided.
Everybody's behaving as though the OP is plotting to wreck their production database. There's nothing in the original question to indicate that this is the case.
What's wrong with tooling around with a toy database on a PC to see what happens?