Add Electrical Powerlines/Towers to SRTM?

Is there a way to add object avoidance like power lines to augment the existing SRTMx data?

Not really, Ardupilot uses quite large grid size, by default IIRC it is 100m, SRTM data is around this resolution. SRTM2 is 30m IIRC.

You could try using your region’s GIS data to generate MP or MavProxy compatible data and including known obstacles. Though keep in mind that the copter/plane will “jump” over them even when significantly higher if it is using terrain following.

I see; thanks!
So if I setup Default Alt. above the highest electrical tower then this will be added to the terrain following, yes?

btw. Is there a place on the MP where I can setup the “Verify Alt” box to be the selected by default instead of having to check it every time?

If you use terrain following the UAV will track terrain at a waypoint altitude (respecting lookahead and its limitations). This can lead to jumps in altitude when encountering tall obstacles. These may or may not negatively affect your mission.

IMHO it is better and more reliable to plan your mission in relative or even AMSL (if you have a system for QNH update and fly long duration missions) frame using GCS to track terrain. Both MissionPlanner and QGroundControl have facilities to check your mission against terrain profile.

IIRC both MP and QGC allow for loading custom maps.