There's no SKILL API to add such markers currently. You can add horizontal and vertical markers using awvPlaceYMarker and awvPlaceXMarker, but there's no API to support adding point or delta markers. Enhancement CCR 317631 is requesting this - I suggest you log a service request and a duplicate can be filed to register your interest.
There is an upcoming feature planned to support interactively adding risetime/falltime type markers on the graph (I did some internal testing recently - the precise release date is not defined yet) - but again, there's no SKILL API for it yet.
With all the work on ViVA that has happened for IC615, the first concern was to provide a good interactive tool with sensible use models; the SKILL API will catch up later.