I would like to use the API to POST hundreds of Sales Order Items to 1 invoice. 

It seems there is no built-in way to convert 100s of Sales Orders to 1  invoice, so I thought that I could just use API to post all of the items from all of the sales orders to 1 invoice. This seems easy enough, but I need a custom field on the items to record the sales order ID #. 

so, my invoice would have hundreds of lines like:

SO ID   |    Item    |  Quantity   | Rate  |  Total 

G1234  |   shirt     |       5          |   10    |   50

I can see how to set up a custom field on the Items. 

1) I would like to know if I can POST any data to the custom field for the item when creating an Invoice. 

2)  If I can post dynamically to the SO ID custom field, how do I refer to it in the JSON?