That's very much possible, I've tried and have it work on various characters, but giving a character the "Summon Follower" skill and having the skill spawn a certain enemy are two different tasks done on different files.
First off, to have the skill spawn a specific enemy, you have to go to the SummonFollower uasset file and change the entries related to the enemy (3 in total, location of the enemy's uasset file, the default function, and the reference function) from the Name Map section in UAssetGUI.
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Like in this example I've changed those entries so they'll spawn the Marauder enemy. You can save the file, though it'll make the Giant Rat spawn the new enemy instead of the rats, or save the file as a new uasset but for the sake of ease I recommend to rename these two entries in the Export Information section to match with whatever name you choose for the new file.
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Here I named the new file as "SummonMarauder" so I changed the entries to match the name.
Next is giving the enemy the skill. Now you'll have to look into the enemy's uasset file and go to an export that starts with "Default__" in the Export Data section.
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You'll have to first see if there's a dropdown named "StartingSkills/Abilities", if there isn't one, then you'll have to add it through the table on the right. You can reference the 'Enemy.uasset' file on adding it, basically just copy and pasting the relevant entries. Then, you'll have to add the reference to the summon skill file through the Import Data section like so:
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The order you add it isn't really important, the -n index on the right just needs to have the index where the "Default__" reference is. Then you'll need the -n index of the one that only has "_C" on the right and use it for the StartingSkills/Abilities entry so that you'll have something like this:
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I know this really isn't a comprehensive guide, but otherwise I might have to write a really long post detailing each step. Don't be shy to ask again if you're stuck.