Here is a way to add a 3rd row seat to the 95-99 legacy or outback. I’m sure 00-04 is similar. I do not claim this to be safe, as the cargo area is not designed to hold people.
You do not need to drill any holes in the car to install this, but drilling will make things cheaper and faster. The cargo area volume is unchanged, as the factory floor is already raised two inches, you are simply replacing the existing styrofoam with the seat thickness. You can buy a seat from littlepassengerseats.com, but they are at least $500 and don’t fold. Weigh increase for thisaddition is very small. Less than 5 extra pounds.
Volvo 850 92-97 or v70 97-00 seat bottom, seat back, and seat bottom hinges. Two seat belts from seatbeltsplus.com(if not drilling) or two Subaru 95-99 center rear lap belts. Some 0.1” thick steel plate(if not drilling) available at lowes or home depot.
To install, removal all the factory floor, and find the attachment points for the cargo tie down, and cross bar. These are the points used to attach the steel plate. The steel plate needs to be cut around the spare tire area. The steel is used as a place to mount the seat back and hinge for the seat bottom. Alternatively, you could skip the steel plate and bolt directly to the floor. The seat bottom, does not require any mods, but the seat back floor is 39” wide, and the space between the shock towers is 37”. Use a circular saw, jig saw, etc to cut the plywood back 1” on either side. Pull back the carpet to do this. The seat itself is not touched. Attach the aftermarket seat belts to the mounting points for the 2nd row seatbelts and run them to the 3rd row seat under the floor. I drilled into the floor and used factory lap belts. They are too short to make it to the 2nd row seatbelt anchor point. As of this writing, I only had one belt. I’ll have to go back to the parts yard to get another. You will need to cut the side carpet pieces to fill the extra space. Also the Subaru cargo area is a little longer than the Volvo, so you need to make a carpet piece between the back of the second row seat and 3rd row floor. If you look close at pictures you will see what I mean.
If you remove the factory cargo tray and spare tire you have a lot of leg room for an adult up to about 5’10”(on legacy, outback has higher ceiling). Any taller, and your head rubs the ceiling. For kids, you can easily leave the tray and tire in place. Head rests are adjustable and need to be pushed all the way down to fold seat. This is a really nice feature. You can see that the passenger side has good leg room, but the driver’s side requires angling legs, because the spare tire bin is not centered. Two colors available from Volvo that I saw; tan and grey. My price for seats, hinges, and one seat belt(I couldn’t find two yet) was about $50 from a pick-it-yourself style parts yard. Obviously, I'm not listing every single detail here, but you can see this isn't particularly hard with a few tools that most people have.
There are other 3rd row seats in the junk yard, 00-05 volvo v70, 00-04 toyota camry wagon, Volvo 240 and 740, but none of them will work this nicely. I measured and looked at all of them.