Leunen has spent the past 10 seasons playing professionally in Turkey, Italy and Germany, most recently with Italy's S.S. Felice Scandone. A native of Washington state, Leunen was drafted by Houston in the second round out of Oregon 10 years ago. His rights were traded to the Clippers in 2016.
Motley signed a two-way contract with Dallas after going undrafted out of Baylor last year. He appeared in 11 games as a rookie, averaging 8.7 points and 4.5 rebounds.
Seibutis was a second-round pick by the Mavericks in 2007. The Lithuanian has been playing professionally overseas for 16 years. Seibutis spent this past season with Spain's Basket Zaragoza.
Dallas also signed guard-forward Ding Yanyuhang, who has played professionally in China for seven years. Ding was with the Mavericks during the summer league last year.