I am adding a second bathroom to an older 1-1/2 story house. I had planned to tie into the existing waste line. The existing toilet is on the first floor and discharges directly into a vertical cast iron pipe which appears to be 6 inches in diameter. This vertical pipe connects in the crawl space to the hub of a cast iron fitting which has two 1-1/2 inch diameter connections just below the hub for the tub and the sink. The line then goes into the crawl space ground and out to the septic tank. There are no cleanouts on the line in the crawl space which has about 30 inches of space between the floor and the ground. The vent pipe extends vertically on the OUTSIDE of the end wall of the house to a point just below the roof.

Is there a practical way to tie a new PVC lines into the cast iron pipe in the crawl space, or would it be better to add a completely new PVC line to the septic tank for the new bathroom?