At the moment, there are no tools in Avogadro to build loops. As @Thomas mentioned, you can construct a variety of helix motifs SS, DS, etc. but not in the context of a loop or origami.
That said, in Avogadro v2, there’s a robust system for plugins to interact with other programs or websites e.g. RNAfold, etc.
You can certainly read PDB and other files into Avogadro after generation.