Someone generated Bitcoin addresses (all unused so far) and added (ECC) the pubkeys plus the pubkey of the genesis address 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa and got a new pubkey (1C1gQwDk2sxEaYVuMjBiuCAYaNw1JE2N3s) and all calculations are correct.

Then he/she/they signed a message. The signature is correct.

To sign the message I need the privkey that I don't have. Is it possible to get the privkey without having the privkeys of the list?