Hi Madhumita,
Following FQEVENTS are available to add an extra line or document at the time of invoicing:
R402 - Add new BP and G/L account items to FICA document. If required, LPC can be printed on invoice using event R410. You can perform additional check on this extra line item in event 0040/0050.
0032 - Add an additional line in finance document. Perform checks in event 0010/0030.
Hope it helps.
Ravi