Can the seat stem be replaced with one that has shock absorber capability?
Yes, you can replace the regular seat-post with suspension seat-post. You need to make sure that the new suspension seat-post does have the same diameter. VoltBike Mariner for example is using seat post with diameter 30.4mm. I personally don't see much value of installing suspension seat-post on fat bike. The extra large tires already provide pretty good cushion when riding.