I don't think you can, just set a 1 char def and forget it exists.

Not  sure  this  will  work,  but  set  a  1  champ  defense  as  recommended  above.    Then  select  that  champ  to  feed  to  another  in  the  tavern,  either  level  or  rank  up.    The  champ  will  be  greyed  out,  but  a  notice  usually  pops  up  asking  if  you  want  to  remove  the  champ  from  arena  defense.    Just  not  sure  if  it  will  allow  you  to  eat  the  last  champ  on  defense.

Good plan here.

(I do it when I want to push an arena tourney, or need to do one for a fusion)