This is not a question but I tried to know how to do that and I noticed the answer is really hard to find. Now, I did it and I can explain how I did it...
I first programmed my programmable xbee module by the common way, with the multilink programmer. My program listen to external commands. One of these is "reset and enter in bootloader" what hapends when I call this is :
app_rst_casuses_t reason=APP_CAUSE_BOOTLOADER_MENU; sys_reset(reason);
After calling these two functions, the module goes in the bootloader.
First connect an xbee module to the computer. Transparent mode, native, right PAN ID, 64bit destination address (module where to load the new custom firmware). As soon as it has been done, open Teraterm, send the requested char to reset the device and enter in bootloader, press 'f'. The module waits on the XMODEM transmission. file->transfer->xmodem navigate to the .abs.bin file to load and ok.
Of course, there would be a better way to do that but this one works...