Yes, you can start entering information for the new period prior to doing a Period End. Period end in AP will warn you if there are batches open or if there are posting journals. However, it will allow you to continue.

Year End will not. Prior to completing the Year End function, all batches must be posted and all posting journals must be cleared.

However, you are not required to run a Period end or a Year end.

Running the Period End function will:
Set Period to date totals to zero for statistics.
Clear paid transactions up to and including the clearing date.
Clear fully invoiced retainage transactions.
Clear batch status info (optional).
Clear vendor history up to and including the system date (optional).

Prior to running the Year End function, you do not need to run the Period End as the Year End procedure will produce the same result. Running the Year End function will:
Move vendor year-to-date totals to last year totals for statistics.
Zero vendor year-to-date and period-to-date totals for statistics.
Clear paid transactions up to and including the clearing date.
Reset batch numbers to 1.
Clear batch status information.
Clear completed retainage transactions.

Prior to running the Year End you may want to print the Vendor List report to have a record of vendors for the year.

Just a final note regarding Ledger. If you post batches for the new year in Payables and retrieve them to Ledger, you will need to Archive the batches in Ledger. You cannot do Year end procedures in Ledger while you have Open Batches.