here is the way you can do it
1- Import Your existing eclipse fie to petrel
2- Export .vol file from OFM database or make vol file with your notepad.
3-Import it right clikcing on "Global Observed Data" in petrel "Input" pane
4-Right click on imported case and "Convert to Petrel Case"
4- Define "History" developmenet strategy
5- "Porsess" tab select Define simulation case
6- Define new simulation case uging this new input data and they export the case ( the PVT and et could be just any model just make one )
7-petrel will export "YourCaseName_SCH in case folder .
8- use this file and included in your original Eclipse DATA file .

9- Hole it helps you

7-