Yes, if you open this form from a main switchboard. Make a button to open the form in edit mode and a button to open it in data entry mode.
Leave the Allow Additions property set to No.
The code to open the form in Edit mode would be:
DoCmd.OpenForm "YourFormName", , , , acFormEdit
The code to open the forn in Data Entry would be:
DoCmd.OpenForm "YourFormName", , , , acFormAdd
("YourFormName" should be the name of your Form)
HTH
RDH