Can one put gaskets between fittings in the routing? Yes.
Can you have gaskets in spoolpieces? It depends on what you mean with spoolpieces, if you mean the spoolpiece functionality provided by SW then I'm not so sure you can because as far as I've experimented with it, it is still very much a work in progress. Then again, I haven't really played with it in the '14 version.
Are you referring to placing a "lined-pipe" route? (example: polypropylene-lined carbon steel) I don't have an answer for you, but I'd like the answer to this if one comes along.
It depends on how good you are at creating configured, design table driven, assemblies of flanges/gaskets/studs-nuts, and their associated sizes/ratings/materials/etc.
We have created custom configurable library assemblies for several companies because there really hasn't been anything native within routing to do the same as we provided...yet?!?