XBee power supply design using a Li-Po 3.7V battery
Yes. Diode's won't be a robust solution, but a small LDO regulator will do exactly what you need it to do. It seems the most current the XBee will use is about 120mA, which is well within a typical LDO regulator's limits.

I am concerned about the battery though. LiPos are large dangerous batteries that are designed for very high currents for short periods. Not a small current for a long period. So, are you designing an IoT type of thing? Or trying to add WiFi to some RC device? In the latter case, awesome!