Use the Insert And Edit Fillets dialog to insert and edit the arcs in a composite curve containing lines and arcs.
Click Curve Tools tab > Segments panel > Insert/Edit Fillets to display the Insert And Edit Fillets dialog.
This dialog contains the following:
Enter Radius — Enter the radius of the fillet.
If you select a specific point or arc centre, the fillet is applied just to that point or arc.
Starting with this curve:

Selecting one point and entering a radius gives:

If you do not select any points or arcs, the fillet is applied to:
Starting with this curve:

On the Selected tab entering a radius produces:

This edits the fillet at
as well adding fillets at points between two lines.
Starting with this curve:

On the All discontinuities tab entering a radius gives:

The fillet at
is not changed.