Hey @Wofrix! Welcome to the forums!
So the skeleton’s bones will need to be modified in another program such as Blender or Maya and reimported. You should learn the hierarchy for the UE5 skeleton and make sure the bones have the exact same setup in order to minimize issues with retargeting.
Regarding the ABP, you have to retarget the animations themselves to that new skeleton. You can do so by highlighting them all and right click > retarget. You’ll want to have a space to put them as it will create new copies of the animations. 
Here’s some documentation! Give it a read and see if it will help!