Hello,
the OK Tech WS add_course operation is in fact calling the Moodle 2.0 API function create_course located in course/lib.php. Indeed in Moodle 2.0 ( but not in 1.9) this function reset to 0 the sortorder value (see line 3677 with the comment '// place at beginning of any category').
I just don't know why it is done that way in 2.0... so unless you comment out that line (with maybe some unexpected consequence later), there is no way to achieve what you want.
Cheers.
Edit : maybe signal this feature to the Moodle's tracker requesting a fix for 2.0.x