How do I weave a field into a BLC core class?

Create an interface for the class that will have the getter/setters:

public interface SequenceSkuPriceDataTier {

    /**
     * set the sequence of tiers (order of applying tiers)
     * @param sequence
     */
    void setSequence(BigDecimal sequence);

    /**
     * get the sequence of tiers (order of applying tiers)
     * @return
     */
    BigDecimal getSequence();

}

Create an @Embeddable implementation class that will use the @Parent to the class that will be woven into. In this class, you will create any/all the field that will be woven in:

@Embeddable
public class SequenceSkuPriceDataTierImpl implements SequenceSkuPriceDataTier {

    /**
     * This is a reference to the price data tier which these SequencePriceDataTier attributes get weaved into
     */
    @Parent
    protected SkuPriceDataTier skuPriceDataTier;

    @Column(name = "SEQUENCE")
    @AdminPresentation(friendlyName = "BillingCyclePriceDataTierImpl_SEQUENCE",
            group = SkuPriceDataTierAdminPresentation.GroupName.PriceDataTier,
            order = 1001,
            gridOrder = 1,
            prominent = true,
            requiredOverride = RequiredOverride.REQUIRED
    )
    private BigDecimal sequence;

    public SkuPriceDataTier getSkuPriceDataTier() {
        return skuPriceDataTier;
    }

    public void setSkuPriceDataTier(SkuPriceDataTier skuPriceDataTier) {
        this.skuPriceDataTier = skuPriceDataTier;
    }

    @Override
    public BigDecimal getSequence() {
        return sequence;
    }

    @Override
    public void setSequence(BigDecimal sequence) {
        this.sequence = sequence;
    }
}

Create a weave class that will embed the class:

public class WeaveSkuPriceDataTierImpl {

    @Embedded
    public SequenceSkuPriceDataTierImpl embeddableSequencePriceDataTier;

    public SequenceSkuPriceDataTierImpl getEmbeddableSequencePriceDataTier(boolean assign) {
        SequenceSkuPriceDataTierImpl temp = embeddableSequencePriceDataTier;
        if (temp == null) {
            temp = new SequenceSkuPriceDataTierImpl();
            if (assign) {
                embeddableSequencePriceDataTier = temp;
            }
        }
        if (temp.getSkuPriceDataTier() == null) {
            temp.setSkuPriceDataTier((SkuPriceDataTier) this);
        }
        return temp;
    }

@Override
    public Address getAddress() {
        return getEmbeddableLazyFulfillmentGroup(false).getAddress();
    }

    @Override
    public void setAddress(Address address) {
        getEmbeddableLazyFulfillmentGroup(true).setAddress(address);
    }

    @Override
    public Phone getPhone() {
        return getEmbeddableLazyFulfillmentGroup(false).getPhone();
    }

    @Override
    public void setPhone(Phone phone) {
        getEmbeddableLazyFulfillmentGroup(true).setPhone(phone);
    }

}

In CoreConfig add the configuration needed to weave in the class:

@Merge(targetRef = "blMergedClassTransformers", early = true)
public List<DirectCopyClassTransformer> mySequenceSkuPriceDataTierTransformers() {
    return Arrays.asList(new DirectCopyClassTransformer("Core")
            .addXformTemplate("com.broadleafcommerce.pricelist.domain.SkuPriceDataTierImpl",
                "com.weave.WeaveSkuPriceDataTierImpl"));
}