Ah, but you live in California. Isn't that like the nepenthes home away from home.
It's not impossible. I grow all of my neps indoors without any special enclosure and many of them have sphagnum growing in the pots.
What I have found works well is to use a larger than normal pot. Fill the pot about 1/2 - 3/4 of the way with media of LFS and perlite and then top dress with sphagnum. The walls of the pot help keep the humidity close to the sphaghnum and it grows well like this. Misting the spaghnum regularly helps in household conditions to avoid dryout.
When I potted this one. I added only a clump or two of Sphagnum. After a few months. The sphagnum had grown up to meet the edge of the pot. Any strand that strayed too far from the lip would dry up.