What does one do if a payment plugin hasn't been developed yet for Beanstream. Does Exp-resso add updates to the payment gateways they support?
You could either develop your own custom gateway integration or contact Exp:resso to get a quote for building it for you. The payment library in Store, CI-Merchant, is open source so you can checkout the existing gateway integrations on github.
Adrian, the developer of Store, says as much in a similar question: Exp-resso Store Custom Payment Gateway / Sagepay Form
If you're familiar with a little PHP then it's not too too difficult to create your own payment gateway to integrate with Store. Beanstream's API seems to be well documented so you could either give it a go yourself or pass it on to another developer to build.
If you're having a go yourself then the best way to start is by taking a look at the existing payment gateways that come bundled with Store. They provide a good example of what you need to do and then you can edit the codebase to fall in line with the Beanstream documentation.
If a payment gateway proves popular enough then it may well be included in a future release of Store but that would go down to demand.
The other answers are correct - we don't generally add new gateways unless they are sponsored.
Our payment library is open source, so if you have PHP development experience it's possible to develop the gateway yourself.
Alternatively, email firstname.lastname@example.org if you are interested in a quote for custom development.