cheap 300zx TT mid-pipes??

quick question on the stock cat. converters there are threaded pipes about 3/4 diameter going into them and about half way down u know so what im wondering is the only straight through mid-pipes with those fittings built in are stillen mid-pipes, i want to buy cheaper ones but they are lacking the fitting unlike the stillen race pipes.. are they something you can delete or move to the downpipes? And also what is it for?