If you have experimental results you can use those directly. You can either program the phase equilibria yourself, in which case your experimental data can simply be adjusted to the mole fraction of water in the liquid phase (the mole fraction of water in the vapor phase is unity, the mole fraction of sucrose in the liquid phase is 1 minus the mole fraction of water in the liquid phase, so you are really only looking for one parameter). This relates directly to the liquid flows and vapor flows as
VF + LF * X_water = F_water
LF * (1 - X_water) = F_sucrose
and
(VF = vapor fraction, LF is liquid fraction, F is compound flow rate at the feed)
F_vapor = VF * (F_water + F_sucrose)
F_liquid = LF * (F_water + F_sucrose)
and of course (VF + LF) = 1.
Again this takes some programming. Rather than explaining procedures of CAPE-OPEN, I can point to the version 1.1 thermodynamic standard specification that explains all procedures. Have a look at the water example at the COCO simulator web site, or the ideal thermo example at the CO-LaN web site.
Alternatively you could use an activity model that is already present in TEA, such as the Margules model (which comes to mind for a 2 compound system, as this has only two parameters;
http://en.wikipedia.org/wiki/Margules_activity_model) and fit the two parameters to your experimental data. Unfortunately an automated fit procedure at this moment, nor are the parameters exposed by the thermodynamics so that you can set up your own fitting automation. So the only way to get the fit done in this case is 'manually'; go into the package configuration and change the parameters. Of course in case you need to fit two parameters, you need to match at least two experimental data points.