It is possible for any finger or for the thumb to slide from a sharp to a natural key and achieve a true legato connection. This finger glissando is notated on the score by a straight line between the finger numbers.
If the thumb slides from sharp to natural key, it is called a finger glissando. This is not to be confused with a true thumb glissando, in which the base of the thumb is used to rock or “slide” from natural to natural or from natural to sharp.
See the Finger Glissando video examples.