How do I import invoices into Zoho books for new clients and create a new client at the same time

We use a 3rd party software to create invoices on that's bundled with our website booking engine.

We want to be able to import those invoices into Zoho books and create new client data as necessary, updating existing client data as necessary.

I can only find a way to import invoices to existing clients.

I'm quite new to using Zoho books and can't figure this out, any help would be greatly appreciated.

kind regards

Jules