I assume you have already gotten yourself a .NP domain name. If not, you can read the process here.
Mercantile only lets you change Nameservers of your domain, however, services such as Google's Blogger and Tumblr, require you to change A Record and CNAME, but not the nameservers. So, if you wish to use your domain name for hosting blogger or tumblr blog, then there's only one alternative : to use third-party DNS service which supports A and CNAME records. Here's what you need to do in order to use your .com.np domain with Tumblr.