Connecting a RF antenna
Don't connect the external antenna to the existing antenna wire.  Antennas are designed for a specific frequency, and changing the dimensions by adding more wire or a cable will probably make the performance worse, not better.  You may be able to connect to the PCB pad (after unsoldering the original antenna), but this depends on the circuit design.  There may be impedance-matching components on the board that are optimized for that original antenna. It's impossible to say for sure without seeing the board and schematic.