Is it possible to add a default .htaccess file to new cPanel accounts?

Yes, just place it in your skeleton directory as you would any other file you wish to have as the default for new accounts.

Where is the skeleton directory located?

FTP to your main domain (if you are a reseller) and you will see it there /cpanel3-skel

Or /root/cpanel3-skel if the server is yours.

I am looking at /root/cpanel3-skel/public_html but the directory is completely empty. Is it supposed to be empty?

Put your files there, those you want to be copied over to new accounts, like the .htaccess file you want to add to new cPanel accounts.

Log into your WHM software and click the "Skeleton Directory" link.

Now read what it says.

Bingo.