Complex sentence: There he became active in SF fandom, including the publication and illustration of fanzines; it was in connection with these activities that he originated his pseudonym, first "Hans," then "Hannes," Bok.
Split into simpler sentences: There he became active in SF fandom, including the publication and illustration of fanzines. It was in connection with these activities that he originated his pseudonym, first "Hans," then "Hannes," Bok.