There's nothing special about wtmp. It's a file just like any other. You can write to it with a output redirection (such as echo "Test" >> /var/log/wtmp) or with a program you've written. All you need are the right permissions; meaning you will probably have to be root.
I would strongly recommend against it. The wtmp file contains a record of all logins and logouts. There's no reason to edit the file unless you've created a new way to login and logout that neither init or login can handle. This is a log file that many intruders want to edit to cover their tracks after breaking into a system. So unless you or whoever the system administrator is don't care about accurate record-keeping, then this file will likely only be writeable by root.