I am trying to sign certificate (X509) using secret sharing. That is shareholders combine their signatures to produce the final signature which will be in this case the signed certificate. However practically from my understanding only one entity can sign a certificate. Therefore I want to know:

1. which entities or data of the x509certificate are actually taken as input to the signing algorithm?
2. Ideally I want this data to be signed by the shareholders and then the final combination will be passed to the X509certificate as valid signature.
3. Is this possible?
4. How could it done?
5. If not are they other alternatives?