to complement some missing data (a small stream currently ending before its mouth in OSM) I want to use an external map, where this stream is present (http://www.geoportal.de/DE/Geoportal/Karten/karten.html), since on all the satelite photos normally available here for editing it isn’t visible. Is this possible somehow? I’m using editor iD.
In another question here I learned, that for this purpose iD has the custom background feature. The problem would be, how to specify the desired map section on the geoportal site. Is here anyone, who does know this?
You MUST NOT use that map as a source for OSM. The site says:
Your second question is no longer relevant given that you cannot trace over the map from that site. However, if you had map tiles from a properly licensed source JOSM would be much easier to customise.
You need to actually go to the place and trace the course of the stream using satellite navigation, if there is sufficient view of the sky, or by measuring relative to already mapped features.