My guess is you want to use Interactive Reporting because you are already using that to retrieve actuals information, or maybe you want to show this data in a nice Pivot or Report section.
You could code the 'extrapolate' logic using a Computed Item in your Results section, then use the Computed Item field in other sections (like a Pivot or Report). i.e. you could calculate in a computed item the % change in volume each year, and use that percentage to forecast next year values. Or, you could code similar logic using MS Excel. Even if you were using a Forecast/Planning tool you would need to build the requirements logic in to a calculation script or business rule.
Hope this helps,
I am using Hyperion IR vs 9
Computed Items will require intervention from me with manually governed growth rates (% increase/decrease). I'm actually looking for someting more sophisticated where there the slope of the growth(or decline) is calculated by a linear equation. Wasn't sure if using Hyperion IR is commonly used in this fashion to make future trend assumptions. I am certain I'm going to be asked by my constituents if it can.
IR is really not designed for what you are wanting. You can Query and get rows of data for Actuals, but there is no mechanism to Add rows for future months.