Ah, yes, a capture card. You have basically two options that I know of, neither is very good.
1 - you MIGHT be able to use firewire if you have a STB outputs non-encrypted video. Firewire is not natively supported by LMCE so it's not going to integrate well unless you create a template for it (perhaps Thom can point you in the right direction or search the wiki). However, you can set it up from within Myth separately, at least as a test. In my case I tried it and while the picture quality was great I was still stuck with the same channels as my tuner card, the encrypted ones are still encrypted over firewire.
It's also not very stable and can cause Myth to lock-up if you try to tune an encrypted channel including changing the STB outside of Myth (ie - via STB remote). Currently I have firewire defined on my MD in my bedroom, but only so I can go into Myth without getting the no inputs available loop and only long enough to back out to the recorded shows area. I do not record anything on it.
2 - wait until something like the HD PVR is fully supported.