This depends on whether you're asking before or after your orders are in production.
For orders that are already in production: It's not possible to combine multiple orders and ship them together once they have been approved. Separately approved orders are dispersed throughout our factory which makes it difficult to combine them once they're in production.
For orders that are still in proofing: We could cancel all but one of your orders and recreate the cancelled line items on your remaining order. This would require you to go through proofing again, but would ensure your items ship together. If you need to add items to an existing order, please contact us, and we'd be happy to help!