can you convert a hardtail mountain bike frame to a full-suspension frame?
Not unless you have a fully-equipped machine shop, no. And even then, the end result might not be much good. It's not just a matter of chopping up a hardtail frame and welding on a few things-- The suspension system, pivot and frame all need to be designed to work right with each other.
No, I wouldn't suggest you do it. There have been some prototype stuff done now and then, but I doubt the bikes would be reliable, of a decent weight, and the full suspension mechanism will most probably suck outright. Geometries are different, and in a full suspension, how everything is designed (where to place the pivot(s), length of the chainstays, seatstays, etc) makes a big difference how the bike works. The only way to do it is, you hold your seatpost, remove it from your hardtail, and then insert it into a full suspension frame. So, while it's a nice idea, it is not a good one, and you won't get a good bike if you work something out.
As previous responders indicated, it's possible to convert a hardtail to full suspension but not very practical. However there is a way to give some cush to a hardtail without adding a lot of weight or costing an arm and a leg. A suspension seat post will take some of the edge off the harsh stuff for normal cross country riding. They start out in price around 30 bucks for the least expensive models and go on up for posts with more adjustablility and travel. A caution though, don't expect a suspension post to turn your hardtail into a big travel, free ride bike.
