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suitescript usually follows the same rules as the ui

if you cant create the record in the ui, you cant create the record in script

If by locked period you mean the accounting period is closed I don’t think it is possible to post anything after closing. 

Though I have never seen it done with a billing. It requires some manipulation to change a payment on an invoice. 

my apologies, i will try to give more context regarding this.
my customer  has a bill that has been created last june. they have approval routing enabled and when the final approver approves the bill today the posting period is set to July. 

They have workflow action script that sets the posting period to the next open period (which is July) is it possible to modify the script such as when you approve the bill, the posting period is june

Sorry if im not quite familiar with accounting terms

the question becomes can you edit the bill from the ui so that the accounting period is june