Can i add a new attachment to an object without checking out and checking back in?

Why is the iteration important? By default all actions in Windchill are only done at the version level. Using the iteration for something is asking for trouble.

Is the object at a released state or at a normal user accessible state?

Do you have admin rights?

As an admin, you may be able to set the state back from Released and then replace content of the attachment without an iteration change, the set state back.

I would seriously reconsider your same iteration requirement. You are losing traceability with that.

As Ben Loosli said in his reply, iteration is needed for the trace-ability and it is very core functionality of Windchill.

If you are looking for less no.of clicks for users to replace the file you can use:  <Document> #Actions #Replace  Action. This will replace the attachment in single click. This action will take care of both check-out and check-in automatically.

I hope this helps.