Can I use a key-derivation-function as the hash function H in SRP?
Yes, you can and use a slow hashing function when constructing the verifier.  I would recommend using PBKDF2, as it is designed for this purpose.

In fact, Wikipedia says:


  $v$ is the host's password verifier, $v = g^x$, $x = H(s,p)$. Using of functions like PBKDF2 instead of $H$ for password hashing is highly recommended.


Thus, you could use $x=\text{PBKDF2}(s,p)$.  Alternatively, you could pre-hash the password with PBKDF2, then use the result as your $p$.