Is there any way to add a new state to a system?

[Sina Porsa]

Hi everyone;
I was wondering if I can add a new state into a model so that I can integrate it's ODE using Forward Dynamics?
Lets say my system has 5 states (x1 to x5). SIMTK generates the ODE s and we can integrate them using FD (forward dynamics)
I want to add a new state (x6) to this system and I know the definition of its derivative (which is a function of the other states) for example:
x6dot = x1 + x3;
I also know the initial value of this new state (x6-initial)
I was wondering if there is a way to add this new state into the system so that I can integrate it?

Regards,
Sina