1. Install Samba Filesharing for Android.
2. Set up Samba and set settings for external SD Card if needed.
3. Open My Computer on PC, find the device in Network, right click on the network folder inside the device that you want to assign as network drive and select "Map Network Drive...", select a drive letter.
4. Open Musicbee, right-click the network drive, select "Manage Folders" and then "Set X: as a Virtual Device".
5. Right-click on the virtual device and go into "Device Settings" to configure.
While transferring files it is best to prevent the phone from going to sleep and keep it plugged-in for optimal Wi-Fi transfer speed. The speed will be much slower than over USB, about 0.7-2 MB/s, mostly depending on the phone.
* This works flawlessly except the phone must be rooted to use Samba, so additional work might be required. Search google to find the xda-developers forum for your phone.
Also:
*If there are no drives listed in the pane on the left-hand side, go into Edit-->Preferences-->Layout and make sure "computer" is selected under left navigator panel. That took me forever to figure out..... Once the drive is set as a virtual device, "computer" can be deselected.