How can I modify equations of a built-in physics model ?

Hello,

I am working on a piezoelectric device and I would like COMSOL to take into account the electrostrictive effect.
I know that it's possible to build your own physics but here I just need to add a single term to one equation.
So I'm looking for an option in a menu, or a .mphphb file that I could modify; but so far I haven't found it.

Do I need to rebuild the Solid mechanics physics? Can I add an equation that override the one COMSOL currently using ?

I really appreciate any help you can provide.

Thank you,

Pierre