WordPress runs slugs through its
sanitize_title_with_dashes() filter function which replaces dots with dashes. Unfortunately the function doesn’t give you any control over that or any ability to change what characters are stripped or replaced.
Periods in Title
Periods in Titles allows you to put periods in your post, pages and category URL slugs. e.g.
Download by clicking the link. Upload to your
/wp-content/plugins/ directory and activate the plugin in your WordPress admin.