How to enter modified conservation equations to comsol?

Hello,

I'm trying to solve a conjugate problem with modified conservation equations.
How can I give my PDE equations to the COMSOL?!
Can I use the mathematics physic in Comsol? I have extra term to add to the default equations.
or Can I write my equations in Matlab and link it to Comsol to solve my conjugate problem?!
or is changing the predefined equations and adding more variable in the equation possible??

Regards