Millwood, 37, spent much of his early career in the National League and has 467 career at-bats, hitting .122 with four home runs and 27 RBIs. He'll start Friday's series opener against the Rockies at Coors Field.
Beavan, 23, has never played in an Interleague road game and will get his first professional at-bats when he starts Sunday.
"The last time I took live BP was my senior year in high school," Beavan said. "It'll be fun. I'm not going up there expecting much, but if I put the ball in play, I think I'll be happy. Hopefully I'll be bunting every time, because that would mean I'm moving guys over into scoring position."
Beavan played third base and shortstop when he wasn't pitching during his prep days in Texas and said he was a pretty good hitter, but noted that has been awhile.
Millwood said he enjoys "feeling more a part of the game" when he's allowed to hit, but his expectations are realistic.
"I'm not scared up there," Millwood said. "But my whole goal is to make sure I can get bunts down when I need to, and everything else is kind of gravy. But at the same time, you can do a lot to help yourself win a game by being able to handle the bat a little bit.
"I feel like I'm adequate. I'm not going to hit .300 or anything like that, but I can put the ball in play and possibly make things happen."
The Mariners' other starter in Colorado will be Jason Vargas on Saturday. Vargas is the Mariners' best hitting pitcher. He hit .354 with 14 doubles and five home runs as a designated hitter at Long Beach State when he wasn't pitching and has a .250 average in 56 at-bats in the Majors.