How to map modified, not matching the destination, source value, using the model mapper?

Question

I have two API entities (EntityA and EntityB) that implement BaseEntity. Both entities have the nullable parameter identifier.

@Value
@Builder(builderClassName = "Builder")
public class EntityA implements BaseEntity {
    String identifier;
...
}

@Value
@Builder(builderClassName = "Builder")
public class EntityB implements BaseEntity {
    String identifier;
...
}

I have POJO as well with externalId parameter. This field value should include the prefix of 4 characters of the source's identifier.

@Data
@ToString(callSuper = true)
@EqualsAndHashCode(callSuper = true)
public class Pojo {
    private String externalId;
    ...
}

I want to map the APIs values to existing POJO PojoEntity, using ModelMapper. However,

1. source getter and destination setter are not match.
2. source's value should be modified.

I can set the value post-factum, on an already mapped POJO.

public Pojo mapEntityAToPojo(EntityA entity, Pojo pojo) {
    modelMapper.map(entity, pojo);
    Optional.ofNullable(entity.getIdentifier()).ifPresent(
            identifier -> pojo.setExternalId(getIdentifierAlphaNumericPrefix(identifier)));
    return pojo;
}

public Pojo mapEntityBToPojo(EntityB entity, Pojo pojo) {
    modelMapper.map(entity, pojo);
    Optional.ofNullable(entity.getIdentifier()).ifPresent(
            identifier -> pojo.setExternalId(getIdentifierAlphaNumericPrefix(identifier)));
    return pojo;
}

private String getIdentifierAlphaNumericPrefix(String identifier) {
    return identifier.replaceAll("[^a-zA-Z0-9]", "").substring(0, 4);
}

But I'm sure, there is a more elegant way, to configure model mapper.
Any suggestions?