In reply to chiragg:
You cannot declare a signal with 0 width, nor can you have an expression with 0 width. However, you can have a concatanation where some of the operands have 0 width as long as the result has non-zero width.
assign out = { {W_USER_SIGNAL==0{32'bz}}, {W_USER_SIGNAL!=0{my_user_signal}} };