redirect subdomain url to subdirectory site

i have WP muti site on my host. i want to give my one of site url which is like sitename.com/subdirectory/site1 to site1.sitename.com

when visitor enter this sub domain site1.sitename.com it will redirect to sitename.com/subdirectory/site1

because sub domian is more rememberable. i want to use subdirectory for my multi wordpress website. i read that subdirectories more seo firendly.

how can i do this ? i m newbie thanks for helping in advance.

[title edited by moderator to help get an answer ]