Can I deduct heteronuclear van der waals parameters from homonuclear parameters in a forcefield?
Found the answer myself. In case someone else stumbles over this, the combining rules for Lennard-Jones parameters of van der Waals interactions are:

Distance parameter: 
$r_{XY}=\frac{1}{2}(r_{XX}+r_{YY})$ or $\sigma_{XY}=\frac{1}{2}(\sigma_{XX}+\sigma_{YY})$ 

Depth parameter: 
$\varepsilon_{XY}=\sqrt{\varepsilon_{XX}\varepsilon_{YY}}$