Product Allocation check in SAP SD

Hi Experts

Would like to ask some queries on SAP SD product allocations.

We have a requirement from customer that sales order should check allocations before it is confirmed. Legacy system will share the product allocation qty to SAP.

My question is how I can update the allocation qty to info structure say S140.

Should I need any custom program development to update the allocations weekly basis, if yes Please share you views on this development and also share the "Update rules for product allocation qty KF".

In short how I can update s140 structure prod alloc qty with allocation data.

Would be great if you could share in detail the solution.

Regards

Sudhakar