Unless the enclosure is utterly bone dry, a ball python won't be seeking excess moisture all that much as they aren't rainforest dwellers (they hail from scrublands, savannahs, and forest edges). But that's what the humidity box is for for when they do have the desire to get their skin wet (especially when in shed, when mine was going through it he spent nearly 3 days straight in his). You'll nonetheless need to mist occasionally if levels get too low as some humidity is needed for them to breath properly and prevent a respiratory infection, but the enclosure shouldn't be drenched either.
And sphagnum moss is an excellent choice. Use it both as a substrate mixed in with coconut fiber and as the humidity box filling. Just ensure the moss in the box in particular is kept damp (but not waterlogged).