In theory, it's easy enough. Just replace the solid axle with a hollow one of appropriate length. The catch -- and of course there's a catch -- is that the new axle has to have the same diameter and thread pitch as your original axle, otherwise you won't be able to use your original bearing cones and locknuts. And there's a good chance that you'll find hollow axles aren't available in the same diameter & thread pitch as your original solid axle.

When my daughters were riding kids' bikes with solid axles, I just kept a small wrench handy to deal with the axle nuts.