I can't figure out how to add URLs to HTTrack projects. (Preferrably by
manually editing the config file and then running HTTrack with --update or
something similar)

I checked the manpage and FAQ and tried searching the forums but couldn't find
anything that seemed relevant.

I never use WebHTTrack because I always spend more time fighting it than
benefiting from it. (It insists on spawning a new browser, dies if I close the
browser it spawns, doesn't like when the single-window-mode browser stays
resident after you close the tab, is a major pain when you don't store all
your archived sites in one place, and imitates a Windows GUI just well enough
to annoy me by breaking all my habitual GUI assumptions) One of these days,
I'll have to try building WinHTTrack against winelib.