Sort of. The Stories series games are not really meant for custom content - they're meant to run light on laptops, so it's really best not to load them full of custom content.
However, you -can- use basic custom content.
I would not use any hacks, hacked objects, or custom objects of any sort - it's a different game with different coding and I could not predict what would happen if you ran into a conflict or something an object needed that Castaway didn't have.
In addition, I think custom accessories (glasses) do not show up in Castaway. Most clothes apparently do show up (I'm assuming ones on custom meshes, or meshes included in Castaway) and I can confirm personally that default replacement skintones, eyes, eyebrows, and hair textures work as well. I've used just those last types and found that was all I needed. Since Castaway is so wildly different from the actual game, custom content of other types would clash with the style anyway.
Downloads for Castaway are placed in:
My Documents\Electronic Arts\The Sims™ Castaway Stories\Downloads