How to add existing page to master page ?

While adding a new web page, select/tick the option "Select master page".

In addition to setting the MasterPageFile as suggested in Vipul's post, you would also need to include the content of the existing web page some_page.aspx within approprate ContentPlaceHolder elements.

Vipul Kelkar15y ago

from your question i understood that you have an existing page say some_page.aspx and you have a master page some_master.master and you need to assign some_page.aspx a master page

go to the properties of the some_page.aspx page and select the master page from "MasterPageFile" property