Leaves: Not allocating leaves on pro rata (period) basis

If a leave type is allows 12 leave annually, when assigning leave policy these are allocated on pro rate basis e.g. for a person who joined in july will be assigned 6 leaves. But if for certain types of leave i dont want leaves to be allotted based on period rather i want that regardless whenever a person joins s/he should have full 12 leaves, how can i do that?