Addings dynamic 301 redirects to the .htaccess file for WordPress from sub-domains to sub-directories.

.htaccess

# You need the following line before the redirects for this to work
RewriteEngine On

# 301 Redirect to subdirectory no query strings attached to the end of the redirected URL
RewriteCond %{HTTP_HOST}  ^site2\.yourdomain\.com$ [NC]
RewriteCond %{QUERY_STRING}  ^page_id=20$ [NC]
RewriteRule ^$ http://www.yourdomain.com/site2/newpage? [R=301,NE,NC,L]

# 301 Redirect to subdirectory with query strings attached to the end of the redirected URL
RewriteCond %{HTTP_HOST}  ^site2\.yourdomain\.com$ [NC]
RewriteCond %{QUERY_STRING}  ^page_id=20(&.*)?$ [NC]
RewriteRule ^$ http://www.yourdomain.com/site2/newpage?%1 [R=301,NE,NC,L]