Yes, you can / you have to because there won't be any other alternative. Companies will operate without HR for 2 to 3 yrs and they feel that one production manager & Finance manager is more than enough to run a company. After 3 yrs the employee related problems gear up because there is no people orientation, welfare, proper statutory compliance, etc., then with lot of dissatisfaction employees go for collective bargaining etc. The real game starts from there onwards, the employees raise disputes, labour department inspects and asks for all sorts of records, appointment letters, documents, confirmation lrs etc. then the organization has to issue the appointment / confirmation letters with retrospective dates.
If the HR joins after 3 yrs even then the HR head wants to keep every thing in order then he / she starts with the job application / testimonials / nominations / etc., then the appointment lrs even then also you have to issue the same with retrospective effect.
Regards - kamesh