Similar to making custom hirelings you only need to add to the description

For troopers
Trooper T1 / Trooper T2 / Trooper T3 / Trooper T4 / Trooper T5

For Offering spectres
Offering Tier 1 / Offering Tier 2 / Offering Tier 3 / Offering Tier 4 / Offering Tier 5

When you unlock troopers you will only be able to spawn T1
The first upgrade adds T2 to the pool
Second upgrade adds T3
Third replaces T1 with T4
And the last one replaces T2 with T5

The odds are the same for every single trooper, an attempt was made to get a specific chance per tier but dnspy is kinda annoying and wont allow it

And as for the offerings spectres
Tier 5 have a 1% chance to spawn
Tier 4 have a ~5% chance to spawn
Tier 3 have a ~9% chance to spawn
Tier 2 Have a ~13% chance to spawn
While all the rest are Tier 1
Overall theres around a 28% chance to get anything other than T1

I kinda want to increase the odds once I figure out how to make it also depend on the player level, if its even possible to begin with