You can give an interval to the ToNurbsCurve method - see http://4.rhino3d.com/5/rhinocommon/html/M_Rhino_Geometry_Curve_ToNurbsCurve_1.htm
or use Trim with an interval http://4.rhino3d.com/5/rhinocommon/html/M_Rhino_Geometry_Curve_Trim_1.htm
To get the parameter values at a certain length, use one of the NormalizedLengthParameter methods, there are many to choose from.
Alternatively, if you want to keep one of the curve ends, you can also use Trim with a length directly - http://4.rhino3d.com/5/rhinocommon/html/M_Rhino_Geometry_Curve_Trim.htm