Sekera's deal is worth $33 million over six years, and Letestu will reportedly earn $5.4 million over three years.
Sekera had three goals and 20 assists over 73 games with Carolina and Los Angeles last season.
The native of Bojnice, Slovakia, has 31 goals and 128 assists in 486 games with Buffalo, Carolina and Los Angeles.
Letestu had seven goals and six assists in 54 games with the Blue Jackets last season.
Letestu has 58 goals and 69 assists in 318 games with Pittsburgh and Columbus.