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Maybe try:
In you CSV file, under"Out of stock" enter 2.
I created a product that allowed out of stock ordering and downloaded the CSV, and the number 2 is in that column.
Try importing just one product with the 2 added and see if that fixes it 
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Maybe try:
In you CSV file, under "Label when backorder allowed" add these words: Ships in 3-5 days. (or whatever your expected shipping time will be)
Try importing one product with the above added and see if that triggers backorder.