best you import the transaction lines into a general ledger journal and post the lines.
you can import the transaction lines using the import module "rapid start" (config packages), map the fields of the import to journal line fields, then import.
the posting of the lines creates a couple of ledger entries, which are then connected to the according customer and other entities.
this is e.g. typical done when importing phone invoice lines.
sales can be handeled by the sales module purchase by the purchase module.
billing is part of both the sales and purchase module.
support: depends on what you need. if you need an issue tracker, a support/hotline mgmt. tool, then you could work with an external tool and connect it with business central for support invoicing.
you could also work within buiness central, create a resource for the support users, use the resource journal and the timesheets. with some customizations you could connect the resource/support user times with the customer, who needs the support, so that you can make the support invoicing also in bc.
