Thanks to Revenat_Doombringer, a method was found to easily convert summoned creatures into player characters.

This is a package of all such creatures ready for use in SoZ via the Party Registrar. All are at the lowest level that they're summonable at, and all level-up exactly like any other player character.

A few of them will work as player characters in the OC and MotB as well, but most will give 'invalid level' errors if you try to use them as your initial character.

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Included Creatures
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Animal Companions
Badger - Level 1
Blue Dragon - Level 18
Boar - Level 1
Bronze Dragon - Level 18
Brown Bear - Level 1
Deinonychus - Level 3
Giant Spider - Level 1
Panther - Level 1
Wolf - Level 1

Familiars
Bat - Level 1
Beetle - Level 1
Cat - Level 1
Homunculus - Level 1
Pig - Level 1
Rabbit - Level 1
Rat - Level 1
Spider - Level 1
Weasel - Level 1

Class Summons
Blackguard Skeleton - Level 10
Blackguard Skeleton v2 - Level 15
Fiendish Dire Rat - Level 9
Fiendish Hellhound - Level 12 (requires Kaedrin's PRC Pack)
Hezebel - Level 14
Koraboros - Level 18
Mephasm - Level 14
Neeshka - Level 20
Nrungnorog - Level 14

Item Summons
Air Elemental - Level 4
Clockroach - Level 4
Earth Elemental - Level 4
Fire Elemental - Level 4
Ivory Shieldmaiden - Level 5
Shamal - Level 10
Skeleton - Level 1
Water Elemental - Level 4

Spell Summons
Celestial Bear - Level 8
Celestial Dire Bear - Level 12
Celestial Wolf - Level 5
Dire Badger - Level 3
Dire Bear - Level 12
Dire Boar - Level 7
Dire Wolf - Level 6
Elder Air Elemental - Level 20
Elder Earth Elemental - Level 20
Elder Fire Elemental - Level 20
Elder Water Elemental - Level 20
Elite Vampire - Level 9
Erinyes - Level 9
Fire Mephit - Level 4
Ghast - Level 4
Ghoul - Level 2
Greater Air Elemental - Level 20
Greater Earth Elemental - Level 20
Greater Fire Elemental - Level 20
Greater Water Elemental - Level 20
Horned Devil - Level 15
Huge Air Elemental - Level 16
Huge Earth Elemental - Level 16
Huge Fire Elemental - Level 16
Huge Water Elemental - Level 16
Imp - Level 5
Mordenkainen's Sword - Level 12
Mummy - Level 8
Mummy Lord - Level 10
Shadow - Level 3
Shadow v2 - Level 7
Shadow v3 - Level 9
Shadow Mastiff - Level 4
Skeleton Warrior - Level 3
Succubus - Level 6
Sylph - Level 4
Vampire - Level 4
Wraith - Level 5
Zombie - Level 2