On a general note, the simplest way to deal with issues like this is referring to the manual.
http://httpd.apache.org/docs/1.3/mod/mod_expires.html#expiresactive
Syntax: ExpiresActive On|Off
Context: server config, virtual host, directory, .htaccess
Override: Indexes
Status: Extension
Module: mod_expires
The two fields of interest it Context and Override. As we can see its OK to use ExpiresActive in an .htaccess file aslong as you AllowOverride Indexes
Update:
To address your need for expire headers. Check out https://stackoverflow.com/questions/1036941/setup-expires-headers-php-apache
Hope this helps:)