So, you want to add the contacts in but not the invoices? But on the other hand you want to save the invoices for a later time?
When importing data (with exception of v3.5 to v4 transfers) you will get the invoices and customer lists, and it will replace your existing data.
I guess it would be possible to use 2 backup files. BUt i wouldn’t recomend it, as it would be time consuming and a high risk of losing data.
Let me know if i have answered your question.
Tyler