You can have an InfoPath form create and update multiple items in a SharePoint list via repeating sections, but it is limited. The steps to get started are below. Keep in mind, I only have 2013 (SP/InfoPath), so steps might not be 100% the same, but I did do this in 2010/2010.
- Open InfoPath Designer
- Create a new form using the SharePoint List template
- Enter the URL of the site/web location
- Create or choose the list
- Check "Manage multiple list items with this form"
This process will create a form for you with a repeating section already in place. As far as I know, you cannot change a pre-existing form into a multiple item editable form, you have to create it this way.
From here, you can design your repeating section as you want. Go into the repeating section properties and choose "Allow users to insert and delete the sections". This will make it so that when they submit, each repeating section and its respective fields will be mapped to an individual item in the SP list.
This method should work fine for creating multiple entries, but requires more work if you want to expand it to then editing them. One method I used before with creating purchase orders was to add a field outside of the repeating section. When doing this, that field's value gets submitted to ALL of your items. Thus, when the form loads, you could then allow a user to pull up all the items on an old purchase order by entering the PO number into that field and use that to query your list to grab all the individual line items for that PO.
Finally, assuming you want to save the form as a whole, and not just as list items, you should still be able to add a submit data connection that publishes the form to a SharePoint library. Keep in mind, if you want to allow editing, that users should still be coming through the list, not the library (assuming you want to keep the values of your list in sync with the values with in the forms saved to the library).