Due to the underneath technology, it's simply impossible to access ssh-agent in SSH Shell, as I've explained the reason in this post:
And:
You don't have to purchase Core Shell, it's free for almost all features in SSH Shell, except Importing and Exporting which requires a premium license. But that's not a problem at all, we provide all SSH Shell users a free one-year premium license, and this should be enough to help you migrate all existing SSH Shell hosts.