Except for the one per faction buildings are there any viable way to increase population growth? I for one has not even encountered a hero with life magic for casting the +1 growth. Where do I find the spell for +1 growth from death shards?

Maybe they could have some technologies that give bonuses to the amount of population harvested from battles and city razing.

Yea, the growth/population is really dependent on where your spawn at when you first start the game. Personally I was lucky and had humanoid mobs for me to farm pop and get pioneers but after it still get super slow and stun the growth cos I didn't have death shard around.

One thing that annoys me is how dead humanoid enemies will add pop to the NEAREST city. Wish we had a way to choose or transfer pop between cities. Always find myself unable to train pioneers from earlier cities cos the pop is all concentrated at the latest city.

I believe the Morrigans' Call spell (+1 growth per death shard) is available from the start if you pick Death magic (based on watching a let's play).

Does the outpost upgrade that provides the +1growth to the attached city work for Undead factions?

Also - it occurs to me that a custom race with Undead blood + Death Magic (for Corruption to turn more shards into Death shards) would be a good idea.

The Life version of Morrigan's Call is Sovereign's Call, which gives a flat +2 growth in the enchanted city. This is way better in the early game when you don't have many, or any, shards, and can remain better in the late game if you have poor luck finding Death Shards. I personally feel that the Empire of the Dead needs Death Worship, and possibly Adepts, for Morrigan's Call to be particularly useful. Alternatively, Gravetouched or Undead could unlock Corruption, which would pair nicely with Morrigan's Call.

Additionally, I think that the Undead trait is rather overpriced, just like Slave Lords is. It would be much better if razing a city were to evenly distribute that city's population over all cities which have available growth capacity, or if it first filled the capitol and then distributed the rest of the population over the rest of your cities, or gave the capitol 50% of the population and then distributed the other 50% over all the other cities you have, and the population from killing Men or Fallen thing would be much more useful if it were applied to Darklings, Wildings, and Mites in addition to Bandits, Ruffians, and other factions. The aspect of the trait which zeroes out unit wages is great, but I think that that was more than balanced out by the fact that the Undead trait turns off normal population growth.

I've played this DLC long enough to know that the growth model, though interesting, is broken, especially at higher difficulty levels.  The easiest and least intrusive solution I thought of is to restore Produce Growth to the Administration tech.  I see it as a fair trade-off for producing upkeep-free troops being forced to use your entire city production/build queue for a good many turns toward growing up each individual city.  The backstory would be easily explained.  The entire city populace comes together for rituals/rites to create new undead.

Another idea is to also allow very minimal continuous growth, but force you to use city population, not just for pioneers, but for troops, too.  Sacrificing living slaves to the dark gods, anyone?  Another idea is to give the mausoleums growth potential.  Etc., etc.  There are a number of different ways to solve the growth issue.  Nonetheless, there must be some way of creating growth without relying on the vagaries of your particular map, ie., the placement of map resources and enemies to prey upon.  And using some of the growth-inducing spells and abilities feels like cheating, since they were obviously geared toward creating more life, not undeath.  If you can't upgrade your cities, all of them, you lose the tech race.  

Or you guys can just build outposts and have them build consulates

Sure the consulates require lots of production but adds +1 growth for each one that gets built for a city.