Do you mean something like this (must be above your # BEGIN WordPress
line)
RewriteEngine On
RewriteRule ^share_this\.php$ /?page=772 [L,QSA]
This will take the request /share_this.php?pid=rk_379061
and rewrite it to: /?page_id=772&pid=rk_379061
, so that the pid
parameter can be accessed.