ok,
I adress it via .sla / xml structure
/SCRIBUSUTF8NEW/DOCUMENT/PAGEOBJECT[num]/
HASSOFTSHADOW="1"
SOFTSHADOWXOFFSET="15"
SOFTSHADOWYOFFSET="15"
SOFTSHADOWCOLOR="Black"
SOFTSHADOWBLURRADIUS="8"
SOFTSHADOWSHADE="100"
SOFTSHADOWBLENDMODE="3"
SOFTSHADOWOPACITY="0.6"
SOFTSHADOWERASE="0"
SOFTSHADOWOBJTRANS="0"