Callaspo, 24, is a switch-hitting second baseman who also plays shortstop and third base. He played 10 games in the outfield for the D-backs in 2007.
"He can play all three positions," Royals general manager Dayton Moore said. "Second and third might be his best, but he can play shortstop. He's a very skilled hitter, as his numbers in the Minor Leagues prove. He's an offensive-oriented infielder."
Last season Callaspo batted .215 (31-for-144) in 56 games for the Diamondbacks with no home runs and seven RBIs. However, he hit .341 (77-for-226) in 59 games for Triple-A Tucson with five homers and 30 RBIs.
"He has a chance to be an offensive force," said Royals manager Trey Hillman. "He's a good defender with everything he gets to.... His range is average, his hands above-average, his arm above-average and his versatility is above-average -- he's comfortable at all three positions."
Hillman said Callaspo has a shot at being the Royals' backup infielder next season.
Veteran Mark Grudzielanek has signed to return as the Royals' second baseman, but Callaspo could be viewed as his successor in 2009. The addition of Callaspo might also give the Royals the option of trading Grudzielanek if the right deal came along.
Buckner, a right-hander, was considered a candidate for the Royals' starting rotation next season. He made seven starts and two relief appearances late last season with a 1-2 record and 5.29 ERA.
For Double-A Wichita and Triple-A Omaha, Buckner was tried in relief as well as a starter. He spent most of the summer with Omaha and was 9-7 with a 3.78 ERA.
"I think Billy is going to be a fine Major League pitcher, but if you're going to acquire anyone with ability, you have to give up ability," Moore said.
Buckner, 24, was a second-round Draft choice out of the University of South Carolina in 2004. He has an excellent curveball and had a winning record, 32-29 and a 4.31 ERA, in his four Minor League seasons.
Callaspo, from Venezuela, is playing winter ball in his home country and is batting .333 in 29 games for Zulia.
Callaspo struck out just 27 times in 490 at-bats for Tucson in 2006 while compiling a .337 average. Last season he fanned just 17 times in his 226 at-bats.
In both 2004 and 2005, he was the toughest player in the Minors to strike out, fanning a total of 55 times in 1,225 plate appearances.
For now, he figures to compete for a backup role with infielders Esteban German and Jason Smith.
On May 10, Callaspo was placed on the restricted list by the Diamondbacks after police were called to his residence because of a domestic altercation with his wife. He was reinstated nine days later and apologized for the incident.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.