Hello TomF,
Ok, within your company's Quickbooks File, you create a number of jobs under the same customer. This transfers to Method CRM as Customer:Job, which are separate entities.
Now you want to do potential jobs and thus wish to use Leads. But the contact field does not get updated when you choose a parent Customer.
And unfortunately, there are no stock screens that automatically do this. The only thing I could see would be to create a new screen, copied from the New Lead screen, and have it automatically populate the contacts from Parent customer when the customer is chosen. If you don't wish for it to happen automatically, you could have a button trigger that.
So yes, customization should be able to achieve what you want.