Is it possible/sensible to add a suspension fork to an old mountain bike? I have a ~1993 Giant mountain bike.  It's not great, but I am a very new and casual rider.  

The bike has a solid fork and it makes for the predictable rough trail ride.  The trails are generally fairly modest (I live in North Texas, a 100' change in altitude makes our ears pop here) and the surface is generally dirt (very few rocky areas).

So, I am thinking that it would be very nice to have a suspension bike.  And seeing that a modest fork can be purchased for a modest price, I am wondering if it makes sense to add a fork to this bike.

This question asked about going the other direction, and there were some cautionary words about changing steering geometry.

So, on balance, is this something that I would be best off to avoid?  I suspect that if I have to replace a bunch of steering parts, the cost will quickly approach that of a new low end bike.
I would consider looking at a used later model bike.The improvements made in the last 18 years are worth the money.A decent fork can run hundreds of dollars not including installation. Check with your local bike shop for used bikes or craigs list if you keep it local so you can see before you buy.Bikepedia is a good reference to make sure you have an idea what the bike cost new. Also check the minimum inflation recommended for your tires and run them softer than you have been,this will cushion some of the bumps.An alternative might be to install the biggest 26 inch tires you can fit in your frame and lower the tire pressure to get more shock absorbtion.Be careful of really lowend bikes from the big box stores Walmart etc. a really cheap shock with no dampening or adjustment is worse than no shock.