No, because I found this issue already existed:
It’s the same problem I encountered, except the message I got about More equations than variables, here are the potential extra equation(s) did not say which equation was the extra one.
So in my example I decided to re-write the equation I needed to take the g(t) variable.
gstate = sys2.states[1]
gparam = sys1.ps[1]
my_idx = 3 # for the INa eq.
LHS = equations(sys1)[my_idx].lhs
RHS = equations(sys1)[my_idx].rhs
equations(sys1)[my_idx] = LHS ~ gstate / gparam * RHS
sys = alias_elimination(extend(sys1, sys2))