Can a humanist adopt the sense of spirituality as held by a pantheist? Yup.
Can a pantheist be a secular humanist? A bit of an iffy area. A pantheist can be secular and also a humanist- but the term secular humanism has much more attached to it these days.
Where does the sense of spirituality come from for a humanist? Depends on the humanist.
Where does the sense of ethics come from for a pantheist? Like most people that aren't heavily tied to dogma, a pantheist would get it from wherever they think is best to get their sense of ethics from. Whether that be a spiritual source, following their gut feelings, or whether it is influenced by certain philosophies. They may even choose to reject the idea of ethics.