I recall reading this story in the early 2000's in an old (90's) Silverberg anthology. The spiritual stowaway, a young girl desperate to escape her boring life in this story has somehow stolen the identity of a cryo (assumed) preserved passenger. The stowaway uses the false identity to gain access to the ship and spy on a crew member. The crew member discovers this and shares his most enjoyable experience on the job, going outside the ship in the thick dark void. Eventually the stowaway is revealed.
This is The Secret Sharer, collected in various of Silverberg's anthologies.
A brief encounter between a young, lonely starship captain and a disembodied female passenger. In the far future, starships are vast affairs where cargo is stored as probability waves outside the ship. As a means of screening new recruits, the youngest and least experienced crew member is always made the captain. Passengers with bodies travel in suspended animation; those without bodies (these are provided at the destination) are stored electronically. One disembodied passenger--young, discontented free spirit Vox--escapes confinement and roams the ship as a bundle of electronic impulses. Alarms sound as Vox accidentally kills a slumbering passenger--so to avoid detection and recapture, Vox superimposes herself on the nervous system of Adam, the captain, with his consent. The pair, whose personalities complement one another perfectly, enjoy a long period of intimate coexistence despite the suspicions of the other crew members. But finally, at voyage's end, Vox must either reveal herself and submit to punishment, or proceed independently outside the ship, where eventually she will fade into random impulses. Tenderly and lyrically set forth, but not even half thought-out, and lighter than air: a nebulous little hymn to the glories of space and the joys of psycho-electronic intimacy.