It is possible to extend the steerer tube on a fork with a steel steerer. Takes some skill though, and quite a lot of trust in ones handiwork to ride the bike later. I've done it by my machining a plug that went inside the steerer and aligned the ends, then welding small patches at opposite sides to avoid warping.

Biggest challenge I see is that for bikes where the suspension is mostly "for show" rear shocks seems to be shorter than for "real" suspension frames. YOu might have trouble finding something fitting.