Fien was placed on waivers Monday and cleared on Wednesday, and thus remains with the organization, though not on the 40-man roster, which opened a spot for Powell.
Marshall, 26, was claimed off waivers from the D-backs on April 4. He's made one appearance for Tacoma, allowing one hit with two strikeouts in two-thirds of an inning.
"There's some history there with both [general manager] Jerry [Dipoto] and [pitching coach] Mel Stottlemyre knowing him a little bit," manager Scott Servais said. "He does it a little bit different than some of the guys we have, more of a sinkerball-type guy with a good breaking ball. Hopefully he can fit in and help us out."
Marshall pitched well for Arizona as a rookie in 2014, with a 2.74 ERA and 54 strikeouts and 17 walks in 49 1/3 innings in 57 appearances, but he had a 7.53 ERA with 16 strikeouts and 13 walks in 28 2/3 innings in 28 Major League outings over the past two seasons.
Fien, a seven-year MLB veteran who signed a one-year deal with Seattle as a free agent, appeared in five games and posted an 11.12 ERA in 5 2/3 innings. The 33-year-old made two scoreless appearances in Houston in the season-opening series. But he gave up a two-run homer to Kole Calhoun in the seventh inning of Friday's 5-1 loss to the Angels, then allowed four runs on two hits and two walks without retiring a batter in the ninth inning on Sunday, when the Mariners blew a 9-3 lead in a 10-9 loss in Anaheim.
That was followed by another run on three hits and a walk in a two-inning outing in Tuesday's 7-5 loss to the Astros.
"Certainly his stuff was very good. He was just maybe a little inconsistent at times," Servais said of Fien. "Obviously the game got away from him over in Anaheim the other day, as far as throwing strikes - that was the big issue there."
Powell was suspended on June 23 after testing positive for dehydrochlormethyltestosterone, an anabolic steroid, in violation of Major League Baseball's Joint Drug Prevention and Treatment Program.
The 24-year-old sat out the final 75 games of Tacoma's season last year and the first five of the current season. He was ranked as Seattle's No. 8 prospect per MLBPipeline.com at the time of his suspension, and he is currently rated No. 24.
Powell posted a .270/.326/.359 line with three homers, 27 RBIs and 10 stolen bases in 64 games with Tacoma last year. He played winter ball in the Dominican Republic and performed very well in Seattle's Major League camp this spring, batting .577 (15-for-26) with one double, two triples, six RBIs and one stolen base in 16 Cactus League games.
• Right-hander Tony Zych was in Seattle on Wednesday to throw a two-inning simulated game. Originally penciled in to throw with Tacoma on a rehab assignment, the Mariners scrapped that out of concern of rain and got him under the Safeco Field roof.
Servais didn't offer an exact timetable for Zych's return but said he is "getting closer." Zych is recovering from offseason biceps surgery that kept him limited throughout Spring Training.
"I can just tell you I feel real good, that's where I'm at," Zych said. "I just want to make sure my delivery stays clean. I've been really working on that. … I felt really good today. I felt like I was finishing pitches really well. The slider was good -- no complaints."
• Two more relievers are progressing on their respective recoveries, as well. After offseason hip surgery, right-hander Steve Cishek is days away from heading out on a rehab assignment, Servais said, though it has yet to be decided where. Fellow right-hander Shae Simmons remains in Arizona and has yet to throw off a mound. He suffered a strain in his flexor tendon during a Spring Training game in mid-March.