[LazyPup]

Since you do not have any threads on the metal pipe you can use a Rubber "Fernco" coupling to connect the trap tot he waste arm. They are inexpensive, easy to install, and it would make a code approved joint to solve your problem. You can get them at any hardware or builidng supply store.

I prefer not to use the flexible P-traps becasue they tend to clog up more than a regular p-trap, however there are times when you have to do what you have to do.