I’m guessing that there’s a reason here that just rotating the actual actor that’s playing the animation won’t work? The fact that you mention “then my character is no longer centered”, at least, suggests to me that the character is starting offset from the origin point, and thus that rotating the actor would rotate the character around that central pivot point (changing their position) versus just rotating the character in place.
I admit, though, I’m having a lot of trouble visualizing the specific scenario.
Are you trying to just change the rotation the character has relative to the rest of the animation? I.e., they start left of the origin, facing left, and you want them to start left of the origin but facing forward, and change none of the rest of the animation? Is the change meant to be universal? You mention changing stuff at import, so I assume so; i.e. the rotation isn’t dependent on anything at runtime, so you want to make one specific change and have that change be the case for every single use of that animation?
If so, I’d say “just redo the beginning of the animation to have the character facing the correct direction in whatever your animation tool was, export the new animation with the change you want, and then re-import it.” Changing the animation you import strikes me as being far less hassle than trying to import an incorrect animation and tweak it at runtime.
But again, I’m not 100% clear from your description on what you’re specifically trying to do.
The above might not apply in your scenario. Maybe you’re trying to change the rotation situationally (i.e. the character could start left of the origin facing left, or left of the origin facing backwards, or whatever, but would always play the rest of the animation the same); in that case, I think you’d have less headache if you just split the animation in two. Have one animation that rotated them to whatever orientation you wanted (or even just manually rotate them), and then play the second animation when they’re in an appropriate position for that animation to play. Etc.
tl;dr: Magic 8-ball says “Situation unclear, ask again.”