Invoice issuers from inside and outside Germany may use the Peppol web service to transmit e-invoices to the federal invoice submission portals. 

However, please note the following:

In order to send e-invoices via Peppol, invoice issuers need a Peppol sender ID. While you are free to choose your own Peppol sender ID, its structure must comply with a schema defined by Peppol; for invioce issuers in Germany, the sender ID is normally based on their VAT ID number. 

In order to submit an electronic invoice via Peppol from outside Germany to an organisation in the German federal administration, the schema of the sender ID must be included in an approved code list defined by Peppol. 

Foreign VAT IDs are often based on a schema which is not suitable for transmitting e-invoices via Peppol and are thus not approved.

Peppol code lists are available here:

Invoice issuers from outside Germany may freely choose from the schemas on this list to create their own Peppol sender ID. 

More information on Peppol is available here and in these FAQs.