Populating Received File name into schema field

I need to populate the Input instance file name in to one of the schema field element and then map it to output schema and send it across.

i can achieve this without writing a custom pipeline that is by property promotion but my root node occurrence is set to max hence i am not able to promote that field.

so can any one suggest me how i can achieve this in custom pipeline component.

i can read the received file name from the context property and assign it to some variable but how can i put this (received file name) in one of the Out put schema field element ? 

Thanks in advance

parashuram