You can't fillet a spline and a polyline, but if you FLATTEN the spline, it will convert the spline to a polyline and then you can run the Fillet command. But, it depends on how the spline was created. If you drew the spline from left to right or right to left, it will affect whether the Fillet works correctly or not. Also, after you fillet the two polylines and then you run the PEDIT command to Spline the polyline, the curvature of the line will change. So there are some issues related to this procedure.