how to add a node or collection element to Sales order extension BO

Hi Experts,

We have developed custom cart rules which will be applied to each line item in sales order based on conditions.

Now on save of sales order I want to save the cart rule id applied for that particular SO (more than 1 cart rules can be applied for a single cart rule).

And this cart rule ids needs to be displayed in SO OWL/report.

I tried adding a node or a collection of rule ids in SO extension , but this is not allowed.

Please help on this issue.

Thanks,

Resmi