l know some subaru outbacks have seats in the trunk, l am wondering if you can install seats in the trunk of a subaru outback if the car does not have seats in the trunk.

you can, and you can buy lap belts to install as well. Its not as comfy but when you need the extra seats at least its there. Call any subaru dealer and you can get belts and clips and even the seat bench or backrest if you want.

You sure can, you can buy the parts needed to latch everything down from the dealer or ideally pull them from another Outback. That way you can just transfer all the parts needed.