Currently there is only an Apache Client HTTP implementation for the DTF HTTP tags. This implementation uses the Java X.509 Certificate Management layer for all certificates related with HTTPS requests. So in order to add your own certificate to this chain you'll have to use Java specific tools for this effect. The certification chain with Java has its keystore located at:
Other locations, such as system level certificates and the ability to even change the keystore location is documented here. To manage the certificates int these keystores you must use the keytool tool supplied with the Java JDK and has some usage instructions here. The usage of the tool is very straightforward if you already have an existing X.509 certificate you want to add to your keystore you just use the following:
keytool -import -alias alias_for_your_ca -file newCA.cer
So on the agent systems where you'd like to issue HTTPS requests you'll have to add this new certification authority to your chain, either at the user level (keystore location mentioned above) or you can do it at the system level for the machine by using the keytool to add this same certificate authority to the chain that everyone using that JDK would trust in (be aware this may not be the most secure way of doing things).