The pin from the LED chip can be soldered to a resistor pin first. Then it can be soldered to other pads on the original circuit by passing it through the circuit board pad. If no additional component pads are available, you can scrape off the copper of this line with a razor blade and then weld it on again. Take care not to come into contact with the side pad lines.
I find that most of the devices use constant-voltage sources as power supplies. Why do LEDs need a constant current source?