ARMA models for non stationary signals
As I've said in the (copious) comments, the signal modeled using

$$
y[t]=A_1(\sin(\omega_1 t+\phi_1)+A_2 \sin(\omega_2 t+\phi_2)+....+A_p \sin(\omega_p t+\phi_p) + z(t)
$$

is stationary.

Your statement i am not interested estimation of parameters,just i am concerned about spectral resolution seems like an oxymoron.  I think what you mean is that you're not concerned with estimating $A_k$ and $\phi_k$, but you are interested in $\omega_k$ for the case where $\omega_k \approx \omega_{k+1}$.

Can you please refine your question, based on this information and the discussion in the comments above?