If you want to know if you can use a directive in a .htaccess in general, you can always consult the documentation. For Apache 2.0, see:
Description: Maps the given filename extensions onto the specified content type
Syntax: AddType MIME-type extension [extension] ...
Context: server config, virtual host, directory, .htaccess
In particular look at the "Context:" part.
In this specific case, yes, you can. You just need to make sure you allow overrides for FileInfo on that particular directory:
You also need to make sure the file EXISTS. If you have a file called test.php you can't just magically request test.phps
In order to solve that problem, you can either use mod_rewrite to force a handler. For example, this rule is in the mod_rewrite documentation as a good example of changing the handler in mod_rewrite
RewriteRule ^(/source/.+\.php)s$ $1 [H=application/x-httpd-php-source]
or you could create a symbolic link but you will need to do that for every file manually (or script it) and for new files.
ln -s test.php test.phps