I'm not sure if this will work for OpenID Connect Provider, but you can do claims mapping for Azure AD B2C provider settings for portals, so the same process might work given similar site settings are being used.
Name:
Authentication/OpenIdConnect/[Federation-Name]/RegistrationClaimsMapping
Description: List of logical name/claim pairs to be used to map claim
values to attributes in the contact record created during
registration.
Format: attribute1=claim1,attribute2=claim2,attribute3=claim3
For example:
firstname=http://schemas.xmlsoap.org/ws/2005/05/identity/claims/givenname,lastname=http://schemas.xmlsoap.org/ws/2005/05/identity/claims/surname,jobtitle=jobTitle