The spirits of the Goetia are portions of the human brain.
The Initiated Interpretation of Ceremonial Magic, Aleister Crowley
I generally take "brain" to mean "mind" here, since I don't imagine Dantalion is just another way to say basal ganglia. He goes on to explain,If then I say, with Solomon: "The spirit Cimieres teaches logic," what I mean is: "Those portions of my brain which subserve the logical faculty may be stimulated and developed by following out the process called 'The Evocation of Cimieres'."
Clear enough.
Hypothesis: Necromantic entities are portions of the human mind that are particularly resourceful in regard to memory and dealing with the past.
I pondered some of the possible symbolism
earlier, which would function as more of a typology than a list of entities. I wonder if "portions of the human mind" might not be better said as "resources of the human mind" to avoid the concept of necessary inclusion, but that's picking nits just now.
In any case, from this point of view, goetic demons and necromantic entities would be close kin. Thus:
Hypothesis: Necromantic entities are amenable to the same methods of access which are more commonly used in Goetic workings.
Unlike the Goetia, however, there's no convenient catalog of demonic names, seals, powers, and other details. Solution? Well, the whole concept of necromancy revolves around the dead, and there are a lot of historical figures whose names and powers/abilities have been well-documented; connect that to the typology (which might be considered similar to Goetic demons' orders or ranks), generate seals (if necessary, many methods available), and there it is.
As a sidenote, since what we're literally dealing with is portions of the mind, there's perhaps no explicit need for the "spirit" to be dead, or even nonfictional. Might break the internal consistency, though, which wouldn't help.
It's of interest that, given the functionally endless number of subjects and the negative aspects of some of the types, there's some latitude in permanent banishing that isn't available when there are only 72 spirits and most have some positive aspects.
There's a bit of crossover concerning three Goetic demons which might be worth exploring:
Bifrons "changeth dead bodies from place to place,"
Bune "maketh the dead to change their place, and divels to assemble upon the sepulchers of the dead," and
Murmur to "constrain Souls Deceased to come before the Exorcist to answer those questions which he may wish to put to them."