[khwcm]

for a elementary reaction,
    K = k_f/k_b

however, for a non-elementary reaction, say Sn1:   
    A + B  C + D
such that
    A           A* + D
    A* + B    C
so is it possible to write a equlilibrium constant in terms of rate constant? thanks alot

[Juan R.]

And where is the equilibrium in your Sn1?

[khwcm]

how about euilibrium exist in both steps?

[DevaDevil]

so I assume you ask the following:

in:   A + B  C + D;  K = k_f/k_b

then what is k in?:
1)     A    A* + D
2)     A* + B    C

am I right in that assumption? ALL equilibria?

then, since you have 2 reactions, you will have 2 reaction equilibria there:
K₁ = k_f1/k_b1
and
K₂ = k_f2/ k_b2

if you want this into one equation you have to rewrite one of them:

K₁ = [A*] [D] / [A]
K₂ = [C] / [A*] [B ]

for K_overall goes: K_Sn1 = [C] [D] / [A] [B ] = K₁ * K₂

so, yes, if they are both equilibria you can combine them into one equilibrium constant by multiplying the individual constants