Yes! You do this by copying the original eCard and making a new eCard for the invitation.
Find your 'Save the Date' eCard in your Sent Cards bin (eCards > Sent Cards) and select 'Copy Card' from the drop-down menu. This will create a new card with an exact copy of the design and message of your original, which you can edit for your invitation.
To add the RSVP event button in your eCard, on the 'Edit Message' screen, select the event you want to create an invitation for in the drop-down list under 'Include Event RSVP'. (If you do not have any future events, it will say so, and display a button to 'Create an Event'.)
Additionally, you can click the button next to the event name to 'Add Event Text to Message,' which will place the event name, date and time in the text box for you to edit further.