Attach Vendor Bill to Purchase Order

Hello,

We have a requirement to attach a Vendor Bill programmatically to a Purchase order when Purchase Order Number is present (in a custom field) on the Vendor Bill. I tried record.attach but get error message -

{"type":"error.SuiteScriptError","name":"SSS_INVALID_ATTACH_RECORD_TYPE","message":"Attaching of record type vendorbill to purchaseorder is not supported."

The idea is to make Vendor Bill appear under Related Records tab of Purchase Order. We can't use Transform since the bill comes into NetSuite via integration.

Any ideas in this is highly appreciated.

Thanks,

Sameer