The Star Trek: The Generation movies were largely released alongside all seven seasons of Star Trek: Voyager. So, because Janeway's crew was lost in the Delta Quadrant during that time, the franchise had to get creative if Voyager stars were going to show up in the movies that continued the events of The Next Generation. The saga managed to pull it off, but none of the official Voyager characters showed up in the first three movies. Instead, two of Mulgrew's costars appeared in 1996's Star Trek: First Contact, but playing different characters from the ones they're most commonly associated with.
Most known for playing the USS Voyager's Emergency Medical Hologram, Robert Picardo had a short cameo in Star Trek: First Contact as the USS Enterprise-E's EMH. Granted, it was a very similar role, but Picardo wasn't playing the same character. Ethan Phillips' First Contact cameo was more separated from his performance as Neelix in Star Trek: Voyager. The actor appeared briefly as a holographic maître d, and was much trickier to recognize without his Talaxian makeup. So, Mulgrew's scene as Admiral Janeway in Nemesis was the first official Voyager cameo in the TNG movie series.
The producers originally wanted Jeri Ryan to return as Seven of Nine in Nemesis instead of Janeway, but Mulgrew's return was deemed more logical.
Voyager & Deep Space Nine Both Crossed Over With Star Trek: The Next Generation (& Each Other)
The 90s Star Trek shows referenced each other in different ways
Because Star Trek: Voyager premiered after Star Trek: The Generation's finale, it was impossible for Patrick Stewart's show to reference Mulgrew's. Thankfully, Voyager was able to include characters from The Next Generation, such as John de Lancie's Q, and it even included Jonathon Frakes' Commander Riker during a short scene in season 2, episode 18, "Death Wish." Star Trek: Deep Space Nine shared the airwaves with The Next Generation for a short while, so the former had an appearance from Captain Picard, and eventually inherited Michael Dorn's Worf as a main character.
Deep Space Nine briefly featured Robert Picardo as both an EMH, and its creator, Dr. Lewis Zimmerman.
Star Trek: Voyager began after Star Trek: Deep Space Nine, and the former's vastly different setting meant any proper crossovers had to happen fast. So, Voyager's premiere, "Caretaker," showed the ship's crew convening at Deep Space Nine's titular space station and meeting Quark (Armin Shimerman). Inversely, Deep Space Nine briefly featured Robert Picardo as both an EMH, and its creator, Dr. Lewis Zimmerman. The show also had a guest spot ready for Tim Russ as Mirror Tuvok in season 3's "Through the Looking Glass." So, Star Trek: Nemesis was just one example of Star Trek: Voyager stars permeating the franchise.