It may be there already, although I'm not sure. Opus's SSH support is based on PuTTY so if you follow the steps you'd perform for PuTTY it may also work in Opus (though you may have to put the key file in Opus's directory instead of PuTTY's, if that's where key files go (I don't know)).
This thread may also help, although I don't know if Pageant is relevant:
[Use Putty's Pageant for SCP authentication)
(I'm assuming that you already know about PuTTY since you're familiar with SSH keys, but if not here's the PuTTY website which may help.)