After some time i had some trial and error....lots of reading i've succeeded in uploading another firmware version to the Stretch 2.0 & Smile P1 using a TFTP server and the U-Boot bootloader.
First you need Plugwise firmware, read this:
"How to Obtain PlugWise firmware + crawler scripts! @ http://domoticx.com/plugwise-firmware-downloaden
Well...i have downloaded already all possible firmware versions, so no need to do it yourself!

Things you need to upload your own firmware image:
1) Hardware: UART > USB stick + some wires
2) Software: PuTTY tools
3) Software: TFTP server
Smile P1:
Connect UART @ http://domoticx.com/plugwise-smile-p1-v ... -uart-ttl/
Upload firmware trough the bootloader @ http://domoticx.com/plugwise-smile-p1-f ... ootloader/
Good to know: Smile has failsave mode till firmware v1.1.9
Stretch 2.0:
Connect UART @ http://domoticx.com/plugwise-stretch-2- ... -uart-ttl/
Upload firmware trough the bootloader @ http://domoticx.com/plugwise-stretch-2- ... ootloader/
Good to know: Stretch has failsave mode till firmware v1.0.38


So, the next level is to compile our own firmware for the Smile/Stretch with OpenWRT


Enjoy!
Some screens: