If I understand your question correctly then this should help:
To Edit Feature Line Curves
If you go to Modify tab > Edit Geometry panel > Edit Curve
Then hit "D" for delete and select the curve you want to turn into a line.
One way: Draw a temporary line from the start to end points of the arc. Use the Featureline editing tools to edit the arc, use the Delete option. Gripe edit the new PI to be at the start of the temp line you made, use the editing tools to Insert a new PI at the end point of the temp line. Erase the temp line.
Are you asking how to get (extract) the curve from a feature line?