New Post: Adding one line to another, is this possible?

Stacking is supported for BarSeries (that use category axes) but not for LineSeries and AreaSeries.
The easiest solution now would be to do the interpolation/extrapolation inside your application, this would also be better for performance (the LineSeries will need to interpolate every time it updates the data)

See also the fork stackedarealineseries:
https://oxyplot.codeplex.com/SourceControl/network/forks/benjaminrupp/StackedAreaLineSeries