After a bit of work, I figured out a way to make it work

Instead of spawning the admiral for the country you're playing as, spawn them for a different country, then after a day or two transfer the nationality from that country to the country you're playing as

I added this code (using the MK3 Bulin from the collab mod into EaW) and it did spawn the admiral with the correct portrait. It would probably be better to use a landlocked nation so they don't have any other admirals to transfer

days_remove = 4

remove_effect = {
JER = {
every_navy_leader = {
set_nationality = ROOT
}
}
}

complete_effect = {
JER = {
create_navy_leader = {
name = "Specialised Bulin Custom MKIII"
picture = "Portrait_UNI_Specialised_Bulin_Custom_MKIII.dds"
traits = { specialised_bulin_custom_mkiii }
skill = 9
id = ALUNI3
attack_skill = 1
defense_skill = 1
maneuvering_skill = 1
coordination_skill = 1
}
}
}