It's something that's actually set by the pool. Most (or quite many of them at least) support manual share difficulty which is usually set in the password field, something like "d=123456" or "x,d=1233456". Check your pools FAQ.

For litecoinpool whole configuration would be like this:

URL: "stratum+tcp://litecoinpool.org:3333"
USER: "username.workername"
PASS: "pass,d=128000"

Hope it helps!