Since there's already a S>D word that extends a single to a double, you could shorten the name above to something like B>N.
Since words that operate on a byte in Forth tend to start with a 'C' such as C@ C! and C, what about naming it C>S ?
AFAIK the C prefix words seems to be more for memory access words than stack manipulating words. B>N looks good to me, possibly B>S fits slightly better with S>D ...