これならOKだ。
assingn  {dumy_cy, madd} =   (maddx >= 13'd3600) ?  maddx - 13'd3600  :  maddx[12:0];

module add_out(run, phase, ct3600, madd);
   input run;
   input [11:0] phase;
   input [11:0] ct3600;
   output [11:0] madd;
   
   reg [12:0] maddx;
wire dumy_cy;
  assign  madx = run ? {1'b0, phase} + {1'b0, ct3600}
 : {1'b0, ct3600};

  assign   {dumy_cy, madd} =   (maddx >= 13'd3600) ?  maddx - 13'd3600  :  maddx[12:0];
endmodule