Hrabal, 35, joined the Steelers mid-way through 2019-20 season and was a member of the winning Challenge Cup championship team.
Mikyska, 26, joins the club from Czech side HC Vitkovice where their play off season has just ended, and has also represented his national side at under-20s level.
Head coach Aaron Fox said: “It’s great having Hrabs back for this Elite Series. He was playing top line minutes in Villacher in Austria and should come back in really good form.
“Hrabs played a huge part in our success last year. He plays a physical game and his compete level is through the roof. He can chip in offensively but prides himself on shutting down opponents’ teams top players and is excellent on the PK. It’s just great to have him back.
“Josef is a skilled centre who is coming off a solid year in the Czech Extraliga. He played mostly on the second and third line in Vitkovice this year and played a very responsible two game while adding 11 points on the year. I feel that is a top league in Europe.
“He showed off his offensive game the year before in Slovakia, finishing top five in league scoring. Josef is extremely talented in tight areas and is a pass first player. I also thought it showed real character by playing in Czech this year instead of staying in a league he was so dominant. I think he will be a great fit and be a big-time contributor.”