All our reference HW designs for 802.15.4 platforms have extra passive components (capacitors and inductors) near to the printed F antenna.
The ~0.5pF C and ~8.2nH L components are used for 3rd harmonic tests. This filter is needed for resonance the 3rd harmonic. However you might use the footprint to implement your own filter for personal tests.
These components are not required for the common transceiver usage. You may not put those components for your design. Please notice that ~10pF Cmust be in your design because it is used for suppressing DC signals.
I do not recommend using a jumper or switch to change from PCB to external antenna and vice-versa (also applies for chip antennas). Either the jumper or switch will bring extra impedance and noise to the system. Then the antenna will not work as good as it should (decreasing the dBm capabilities). A Zero-ohm resistor can be used as a “wire link”. If you want to use the external antenna, you need to change the zero-ohm resistor.
Theoretically you won't have any loses using a zero-ohm resistor, but we know that nothing is perfect in the real world. The loses you will have depends on the resistor quality (depends on the materials, fabricator, etc). Unfortunately, we have not characterized the resistor loses in dBm. Our ZigBee hardware expert recommends that you may use a 10pF capacitor instead of a Zero ohm resistor. The advantage on the capacitor is that when you are in soldering phase, the resistor material may get noise from the process and generate a different ohms characteristic, while the capacitor will not have any problem. However any of the two possibilities will work well for your design.
Please feel free to add more information about impedance matching and similar topics.