tapping in to main sewer line for addition?

first lemme say this forum is great and a huge help as I sort out details of a planned addition to my house!

you guys were good enough to help with how to extend the heat, now I have a question on tapping in to the sewer line for the bath and laundry rooms I'm planning.

and apologies if I get some language and details not exactly right as I am definitely not a professional!

so first the house is old and there are two cast iron sewer lines -- one for the upstairs bath and one for the kitchen. the new addition will connect to the kitchen and essentially sit on top of where the kitchen sewer line is.

it exits the current stone foundation just below grade and slopes out to the street. I'm planning a crawlspace foundation so i can route the plumbing w/o the expense of a full basement. can i tap in to the current cast iron line for the new bath and laundry drains?

should I replace the cast iron under the addition (and back to the original foundation) with PVC and use a coupler to connect to the cast iron as it exits the new crawlspace and out to the street?

would rather not replace the line all the way out. thx again.