This paper develops an exact algorithm for the identical parallel additive machine scheduling problem by considering multiple processing alternatives to minimise the makespan. This research is motivated from an idea of elevating flexibility of a manufacturing system by using additive machines, such as 3D printers. It becomes possible to produce a job in a different form; a job can be printed in a complete form or in separate parts. This problem is defined as a bi-level optimisation model in which its upper level problem is to determine a proper processing alternative for each product, and its lower level problem is to assign the parts that should be produced to the additive machines. An exact algorithm, which consists of the linear programming relaxation of a one-dimensional cutting stock problem, a branch-and-price algorithm, and a rescheduling algorithm, is proposed to find an optimal solution of the problem. The experimental results show that the computational time of the algorithm outperforms a commercial solver (CPLEX). By examining how the parts are comprised when the processing alternatives are optimally selected, some useful insights are derived for designing processing alternatives of products.