Any easy way to set up Multiple subscriptions?

Issue Summary

Is there any way to show a single subscription which is both monthly and yearly and the user can choose whether they want to choose a monthly or a yearly or any 'n' months subscription.
The only way i thought of doing this was create 2 subscription of which is monthly and yearly. But that is too much of a work when you will have 100 of subscriptions.

I thought of editing the module. And as i can see the only variable that we can reference and store our data is $custom and which is only 32 Bytes and its attached a uid and sometime a nid.

So my question is should i attach my own variable to $custom array and map it with an id on my local table or is there any other way to do that?