You can reuse the profile of someone who used to work for you and is returning to your organisation.
Simply go to People > Our people, use the filters to show those who have Left and select your person. Then edit the Summary tab and change their status back to Current.
You should then click reset password. This will send the user a temporary password via email and they'll be required to set a new password when logging back in for this initial time.
One last thing to consider is whether you wish their two periods of employment to count as continued service. If you do, then you should leave the person's join date as it was on the profile. In this way, any leave allowance based on length of service will be unaffected.
Separate terms of employment?
If they are to be considered two distinct periods of employment, the join date can be amended to reflect the new join date for the new period of employment. A pro-rata adjustment will be made for the leave allowance, based on their new start date. If the new term is considered separate, all leave totals will be set to zero.