Hello,

I have started playing this game with a friend Multiplayer and we have played so much that I have a quite high level character. Problem is, I would like to convert that same character and use it to start the Singleplayer campaign.
Is this at all possible?

Thank you!

Short answer: No
Long answer: Technically it's possible but would require setting up some tools and some knowledge of the game/code
If you are high level, you won't enjoy single player at all as it only has normal difficulty = you are going to 1shot everything on your path = what's the fun? Play singleplayer once normally to see the quests then forget about it ;)

I'd say play single player several times still, until you're sure you've seen all the different quests :)

Or use devilutionx with a flag that enables all quests :P