Sure, it's possible. In fact, this is not specific to leachate at all and is applicable to the generic situation where one needs to model a toxic/inhibitory effect.
You will probably need to add a toxic or inhibitory component (e.g. S_Tox) to the component vector of your modified category (e.g. ASM1_Tox). Assuming you don't want to do all this manually, you can add a component through the "Manage Components" button in the Matrix Editor.
Then you can certainly define a state variable (at the category level, so again: through the Matrix Editor | Variables tab page) - as well as the relevant parameters used in the expression of the new state variable.
And finally: include the state variable in the kinetic expression of the relevant processes.
