Frank Arnesen and Martin Jol's only new signings in the current transfer window have no Premiership experience, but both completed successful trials with the club last month.
Hallfredsson, signed for £500,000 from FH Hafnarfjordur on a two-and-a-half-year contract with the option of a further year was recently named Iceland's young player of the year.
Czech Under-21 David Limbersky, who was on loan at an Italian Serie A club last season, has been signed until the end of the season from Viktoria Plzen. The right winger trained with his new colleagues for the first time on Thursday.
"I have always dreamt about playing in England so I gave everything to the trial and it paid off. The coach promised me that I would get my chance," said Limbersky.