Reading pair Sean Morrison and Jordan Obita have both signed new contracts at the Madejski Stadium.
Ipswich defender Aaron Cresswell has signed a two-year extension to his contract.
Dawson was released after 10 years at the club and subsequently joined former side Scunthorpe.
The Iron have a League Two fixture at Bristol Rovers on the same date as the Betis match, August 10, but Dawson has been given permission to play in his own testimonial instead.
The 34-year-old played 317 times for Hull and was part of four promotion-winning sides.