Hi Abhihsek,
I don't think there is any standard way to cater your requirement.
However, enhancing event R433 will enable you to cater your requirement.
In this event, SAP makes available the contract account A/R and A/P documents, including the allocated document numbers created in IS-U invoicing.
In here, you will get the new payment plan doc no. and then based on that, you can store in the print container for the use by the payment plan welcome letter.
Hope it helps...
Thanks,
Amlan