Psychic Sorcerer Bloodline
If you desire to play a psychic spellcaster without actually playing the Psychic class, you can try the Psychic sorcerer bloodline. With this, you gain psychic spellcasting, you gain a few psychic-themed bloodline abilities and spells, and can freely pick your spells from the wizard/sorcerer list.
I will suggest you go this route if you want something closer to the Necromancer wizard spell school, as there are very few undead-themed spells on the Psychic spell list.
The Spiritualist has several archetypes that are thematically fitting for a necromancer spellcaster, especially so when you consider they have a ghost as a pet (their Phantom). To name a few archetypes:
- Necrologist: The vile spiritualists known as necrologists reach farther toward the Negative Energy Plane, however, and for whatever evil purpose, they bring back malevolent spirits whose passions have turned to vengeance and hate for all life.
- Geist Chaneller: Incapable of manifesting in ectoplasmic form and devoid of human emotions, these strange entities are much closer to ghosts than most other phantoms.
- Usher of Lost Souls: Ushers of lost souls are spiritualists who focus on bringing the souls of the dead to their final judgments at the end of the River of Souls. To this end, they specialize in destroying undead and in laying haunts to rest.
The spiritualist spell list also contains several necromancy spells related to undeads, like Animate Dead, Boneshaker, Undead Anatomy and Create Undead. Those spells are all missing from the Psychic spell list.
Haunted Psychic Discipline
The Haunted discipline (from Haunted Heroes Handbook player companion) should be thematically fitting for a necromancer-type psychic character. You gain more powers related to handling spirits and haunts than raising undead though.
Your powers interact with the spiritual world, attracting the attention of minor spirits and undead.
If you plan on using psychic spells against undead, I will suggest you take the Will of the Dead phrenic amplification, so they can be affected by mind-affecting spells instead of being immune, and thus, you can play some sort of enchanter of the dead type of character.
This is the only published discipline/archetype that is even close to a necromancer for the Psychic class. Discarding this option will lead you to other classes that will use psychic magic and have different mechanics and spell lists.
To be honest, personally, I wouldn't even consider the Psychic class, as they are thematically tied to mind, telekinesis and astral themes, not undead and necromancy. To become a proper necromancer, an archetype or discipline would first have to address the spell list and add more undead-themed spells ot the class. This option is here simply to address the class.
Another option is adding necromancy spells to the Psychic spell list, by making a samsaran character and (hand) pick a few necromancy spells with the Mystic Past Life racial trait, and with this, you could pick spells from the Spiritualist spell list and add to the Psychic spell list:
Mystic Past Life (Su) You can add spells from another spellcasting class to the spell list of your current spellcasting class. You add a number of spells equal to 1 + your spellcasting class’s key ability score bonus (Wisdom for clerics, and so on). The spells must be the same type (arcane or divine) as the spellcasting class you’re adding them to. For example, you could add divine power to your druid class spell list, but not to your wizard class spell list because divine power is a divine spell.