Is it okay to submit a modified journal paper that is still under review to a conference?
From two responses in the comments, submitting to a conference after a submission to  a journal would likely have policy violations from both the journal and the conference. 

(#1)


  You would probably violate the journal's policy. Usually, you are
  required to declare that the work has not been submitted/published
  anywhere else. If it has, then you need to note the differences.


(#2)


  you might also violate the conference's policy: The same rules apply.