You can see this in The Bean Stalker. It contains a line

<SYNTAX CLIMB ONTO OBJECT = V-PERCH>

"ONTO" is already defined in the library as a synonym for "ON". However, this line causes it to be defined a as a new preposition

{{
W?ON:: .ZWORD "on"
.BYTE 8,250,0

W?ONTO:: .ZWORD "onto"
.BYTE 8,243,0
}}

But it doesn't appear in the preposition table either:

{{
PRTBL:: .TABLE 46
.WORD 11,W?IN,PR?IN,W?UP,PR?UP,W?DOWN,PR?DOWN,W?THROUGH,PR?THROUGH,W?ON
.WORD PR?ON,W?AT,PR?AT,W?OFF,PR?OFF,W?INVENTORY,PR?INVENTORY,W?WITH,PR?WITH,W?TO
.WORD PR?TO,W?UNDER,PR?UNDER
.ENDT
}}