IF its a "temporary staff" why do you give a Appointment Letter -
Better frame a Training / Apprenticeship Policy (Include the objectives, procedures, manual etc.)
Issue Letter of Apprentice to temporary employees , period being 6 months or 1 year , depending on the nature of the work.
Review thier performance after the prescribed period, issue Letter of Appointment , as permanent employees.
Temporary employees are not on the rolls of the company , and should not be subject to / under purview of any contract (Letter of Appointment)
Once they finish apprenticeship we are bound to bring them on roll basis thier performance
Regards
Sriram