A few things:
What type of Necromancer were you looking for? IE A bad touch debuffer type with fears and curses? Or are you looking for something more minion-mancy focused with lots of pets? Or something in between?
Why Bard? What does this get you? Inspire Courage doesn't work on mindless undead which is the bulk of what you will be using, and doesn't synergize at all with Sorcerer, Wizard, Oracle, or Cleric, which are the most popular Necromancy classes.
Is this for an Adventure Path? Pathfinder Society? Home-brew campaign? What's your book limitations? All/some/core/hardback only? What's your character creation guidelines?
Answer all these things and you'll get a lot more feedback from the community.
As for your primary question: I honestly don't know. You could Resurrect someone then kill them and use Trap The Soul, though that seems a little wasteful in terms of spell components.
is there a way to trap a -already dead- soul?
Depends. For mindless undead, the soul has already gone to Pharasma. But sentient ones usually have their soul still with them, and so should (per the fluff) qualify as "recently dead" where trapping souls is concerned.
The desired effect is to place it into a zombie/corpses body.
Willingly?
The closest thing I'm aware of for this is the JuJu oracle mystery, which binds wendo spirits to zombies.
I agree that the Juju Oracle might fit the bill. I also suggest looking at the Spiritualist class in the Occult Adventures playtest. If you're going to take Bard levels I suggest looking at the Dirge Bard archetype. You can pick up a few select Necromancy spells that way, and it allows your mind-affecting spells to work on undead.
It is true that your Inspire Courage wouldn't help your undead minions, but if you decided to focus on Dirge of Doom instead and armed your minions with Cruel weapons (or amulets) your enemies might end up seriously debuffed. You also eventually get the ability to make dead enemies get up and "dance" for you, fighting as skeletons or zombies.
Depending on the game you're playing in it might be really nice to have a partner in crime, perhaps a Witch who could apply even more debuffs and heal the undead if needed. With the right patron the Witch could also help build the undead armies.