Yes, you can.
Create your mod in the appropriate GalCiv3\Mods or GC3Crusade\Mods directory, depending on which version of the game you have. Call it what you want, then create a directory called game in it.
Go to the C:\Program Files (x86)\Steam\SteamApps\common\Galactic Civilizations III\date\Game directory and find the file MercenaryTierDefs.xml. Copy this file into the game directory of your mod.
Open this file in your favorite text editor. The three tiers are listed there with the amounts in each tier given as <Amount>xx</Amount> where xx is the number that will appear. Change the values to the amount actually in the game to get all of them to be available.
The actual Mercenaries are defined in another file, but they don't matter for what you want. All you need to know is how many Mercenaries are defined in each tier. Here are the numbers.
Tier 1 = 26
Tier 2 = 24
Tier 3 = 22
For your mod, use these numbers in place of xx in the appropriate tier to get all the Mercenaries to appear.