Consolidate Subscription Lines into Single Contract

Hi,

We have requirement to create only a single contract for the customer subscribing for the software lines for their device.

Since our billing would be cumulative in nature meaning ,as in customer keeps ordering more subscription quantity they should be billed from next month for the more quantity till date

Eg. First month billing 1 qty ,if in mid of month more qty ,then next month billing 1+ more like wise.

So we want to always create subscription line for the same item /customer into a single contract instead of creating a new one as in the sales order getting placed.