Oracle 11G undo tablespace

BLUF: Can I create an undo tablespace with a non-standard block size and assign a specific user or session to use it?

DETAILS: I've got a 2 node RAC cluster running Oracle 11G. One of the 3rd party applications is running out of undo extents (max # of extents for undo segment . . .). I have no access to the source code so I cannot change the commands it executes. I'm thinking my only solution is to create an undo tablespace with a nonstandard block size. Our current block size is 16k. Then, either make the application user always use that undo tablespace or perform an "alter session *session_number* set undo = bigblock_undo"? Or