adding a yagi antenna to a bluetooth dongle
A Yagi antenna for bluetooth may increase your gain. Keep in mind that Yagi antennas are directional antennas so if you move your target around you will have to move your antenna.

In order to use an external antenna, you would have to take following into consideration:


Use a coaxial cable with the same impedance as your antenna and your final amplifier at bluetooth end.
Cut the trace on the PCB that acts as an antenna
Try o match the feed point impedance and the impedance at source by minimally separating the coaxial conductors (center and shield) at either end.
Make sure you account for signal loss in the co-axial cable. Use a better cable if you want to reduce the loss.
There could be more points to consider apart from the ones above