Embed Iframe is a plugin that will let you embed iframe – an HTML tag that allows a webpage to be displayed inline with the current page, in a WordPress post. Although an iframe can lead to a complicated website, it can be very effective when used appropriately.
Utilización
Use following tag to insert another page in post using iframe
[iframe url width height]
e.g.
[iframe http://www.example.com 400 500]
Te puedes descargar este plugin para utilizarlo en tu instalación autoalojada de WordPress.