Normally the waste would flow by gravity to the septic tank. The heavy solids sink to the bottom and grease and oils form a layer on top of the liquid in the septic tank. The baffle and tee allow only the effluent to flow by gravity out the end of the septic tank. The effluent usually flows by gravity into a pump tank. Then you would have an effulent pump that pumps the liquid to a distribution box (d-box) or pressure manifold etc. and then to the drain field. In your case you could pump to the existing septic tank. There are other ways to do it but I would do it this way.