I am afraid that currently this is not possible, there is feature request logged, you can vote on it here:http://feedback.telerik.com/Project/153/Feedback/Details/100702-encrypting-the-ldap-username-and-password-in-the-securityconfig-file.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items