The CDF of $K$ is 
$$ P(K \le n) = P(S_n > T) = \int_{T}^\infty dt\; f_n(t)$$
The CDF of $S_K$ (for $s > 0$) is
$$\eqalign{P(S_K \le s) &= \sum_{n=1}^\infty P(K = n, S_n \le s) = \sum_{n=1}^\infty P(S_{n-1} \le T, T < S_n \le s)\cr &= \sum_{n=1}^\infty \int_0^T dt\;  f_{n-1}(t) \int_T^s dr\; f(r-t)}$$