The first thing you need to look at is. Do the Subaru seats have the seat belt buckle attached to them? If not then you probably should look at some different seats.
Then check the width of the runners between the Navara and Subaru seats and the height of the Subaru runner from where it bolts to the floor to the top of squab (the bit you sit on)
If the width is close then you might be able to adapt them.
The Navara seat is a PIA because the outside runner will unbolt from the seat frame but the inside runner is totally different in construction. It is part of the seat frame and the seat belt attaches to that side as well.
I haven't looked at Subaru seats so have no idea how the runners attach. You might be able to cut the legs off the Navara runner and weld to the Subaru.
However, and this is the biggy,You MUST get the modification engineered. Talk to an engineer before you do anything else. Lots of people will say dont worry about it and, from a structural point of view, you or who ever is going to do the work might be quite capable of doing a good job, but if you don't have it passed by an engineer and you are ever involved in a crash then you will more than likely be RS. Most assessors would look at the seats and ask for the engineers report. You don't have one then they will wipe your insurance. Imagine if you have just written off Lambo or something and suddenly no insurance or worse injured someone. Even if the crash was not your fault, you would still be wiped by the other vehicles insurance because you were driving an illegally modified vehicle.
So if you still want to do the conversion talk to an engineer who is an authorised RMS engineer before you get too carried away.
I haven't changed the seats in this Navara because of the hassle(and cost) involved in making it legal. Years ago when I was young and foolish, I fitted Recaro seats to several vehicles I owned including a Datsun 720 ute and later a D21 Navara with no thought to getting it certified.
Looking back I consider myself to be extremely lucky that I never had a crash.
It might actually be cheaper to get your existing seats re trimmed with better padding and support than going the conversion route. When I was looking at this a few months ago I was quoted $500 for the eng report and $100/hour for consultation, inspections etc.
Something I just remembered as well. The Navara seats have a pressed steel pan under the foam of the squab. Its quite thin and in my 2001 model the drivers side was broken around the mounting bolts from fatigue. Something to keep an eye on. Easy to check by using a mirror and having a look under the seat where the squab bolts to the frame.
Hope this helps
peter