Is there a way to add playernames, or IDs in the settings file to exclude them from being purged?

You can grant them permissions that will ignore them

How? I could not find a template/example/documentation anywhere on the umod site

Create a new permission entry in config and set lifetime to "none". Read plugin description here on uMod ;)

Hm, where?
I cannot find a link to the description on:
https://umod.org/plugins/auto-purge

It's right on this page ;) How-To section.

How-To

Permission is a permission. Can be empty to allow for everyone.

Lifetime is a time between player last seen and entities removal in format 1s1m1h1d. Use none to disallow the purge of entities for this setup. If unable to parse lifetime, defaults to purge disallowance. Purge disallowance is the highest priority, so in case at least one of the guys has it, entity isn't purged.

I cannot see a way to exclude a player, or players, or a group here.
I was not able to find a Auto Purge-Permission in the code/permission-manager(ingame)
Do i have to put a not specified permission-name like "purgeme" to the Permission-Setting like:

{
  "Purge Settings": [
    {
      "Permission": "purgeme",
      "Lifetime": "none"
    }
  ],
  "Purge Timer Frequency": 300.0,
  "Last Seen Timer Frequency": 300.0
}

And grant everyone, except the persons i want to exclude, this permission?

Use none to disallow the purge of entities for this setup

Merged post

You should grant this none permission to persons you want to exclude.

I changed the "Lifetime" to "8d" for our server.

{
  "Purge Settings": [
    {
      "Permission": "none",
      "Lifetime": "8d"
    }
  ],
  "Purge Timer Frequency": 300.0,
  "Last Seen Timer Frequency": 300.0,
  "Debug": true
}​

The sentence in your How-To

Use none to disallow the purge of entities for this setup

is context of the "Lifetime"-Setting, where i replaced "none" to "8d", so i was not aware that you want us to know that "none" is the name of the permission for the "Permission"-Setting.
But nevermind, it does not work anyway:

(15:39:30) | Missing plugin name prefix 'autopurge.' for permission 'none' (by plugin 'Auto Purge')

So i changed the permission name to "autopurge.none" right now.

{
  "Purge Settings": [
    {
      "Permission": "autopurge.none",
      "Lifetime": "8d"
    }
  ],
  "Purge Timer Frequency": 300.0,
  "Last Seen Timer Frequency": 300.0,
  "Debug": false
}

I guess i have to create two brackets now, for lifetime-"none" players and one for lifetime-"8d"-players?

Like this?

{
  "Purge Settings": [
    {
      "Permission": "autopurge.PurgeMeIn8Days",
      "Lifetime": "8d"
    },
	{
      "Permission": "autopurge.ExcludedFromPurge",
      "Lifetime": "none"
    }
  ],
  "Purge Timer Frequency": 300.0,
  "Last Seen Timer Frequency": 300.0,
  "Debug": false
}

I try to give admins the Permission "ExcludedFromPurge"-Permission.
All other players(group=default) will be granted the "PurgeMeIn8Days"-Permission.
I hope this works.

Yes, this should work now.