Thank you for posting your query on Microsoft Q&A, from above description I could understand that you are looking for advisory on if you could use key value pair of one certificate to renew another self imported certificate.
Please do correct me if this is not the case by responding in the comments section.
A certificate is a versioned object. If the current version is expiring, you need to create a new version. Conceptually, each new version is a new certificate that's composed of a key and a blob that ties that key to an identity. When you use a nonpartnered CA, the key vault generates a key/value pair and returns a certificate signing request (CSR).
Each new CSR that you create has a private key, which has to match when you merge the signed request. Hence, it's important to merge the signed CSR with the same CSR request that you created. Otherwise, the key won't match.
So the answer to your question would be the 2nd option you mentioned i.e., manually set B's policy, e.g. Type of CA to be integrated CA, lifetime action as auto renew, and configured as 'Reuse Key on Renewal', then create a new version by generate.
Please "Accept the answer (Yes)" and "share your feedback ". This will help us and others in the community as well.
Thanks,
Akshay Kaushik