Tig, thanks for your so fast and explicit reply!
I know what fillet tool can do (and I didn't entirely read your the help file ).
Truth is that I never tried to extend an arc... but I think that's a SU limitation, knowing that they are approximated by segments.
But what I forgot to add to my previous post is that ArchiCAD "adjust" tool can be applied to more that one 2D entity: one can select as many lines or arcs, etc. at once and they will trim or extend all together and, if boundary line doesn't exist, one can draw an imaginary one.
I hope this make sense.
Actually, a set of real 2D Tools are something I was "officially" asking each time I was in the beta tester team, from @Last to Google, and yours are a major step forward.