How do I slow further cracking in wood outdoors?
You don't need to fill in the cracks to prevent further cracking. You need to seal the wood to prevent further cracking/checking. Moisture is wood's undoing, and you want to keep the level of moisture in the wood consistent year-round.

Almost all wood sealants are aimed at doing this--paint, stain, poly, etc. For outdoor furniture, poly-based stains are REALLY good, if you don't mind the slightly rubbery feeling they can leave. For stairs, look into decking products, some of which are poly-based, but are formulated to allow you to walk safely on them.

Now, if you have big cracks, you might get snow and ice in there, and the ice could wedge the wood further apart. Depending on how big the cracks are, you can fill them with epoxy resin, and then seal the whole thing with paint/stain/etc. The stain won't sink into the epoxy and color it, so you'll need to mix some colorant into the resin as you apply it.

If the cracks are even larger, you might consider using something like a Dutchman (bowtie-shaped piece of wood) to hold the two pieces from splitting further apart. This takes some skill and either a router or chisel and a good eye.

If the cracks are enormous, you may just be better off replacing the wood where it has split. Make it match as best you can, and then make sure to put a good coat of sealant on both old and new wood.