Hi sbaldwin,
I've done some research on this topic, and it seems like there are quite a few people interested in combining both WordPress and OpenCart, but it doesn't seem like there are any solid or any official plugins that will do this.
Here are a few links I found that you may find interesting for review:
OpenCart + WordPress IntegrationCombining OpenCart with Wordpress
Because there doesn't seem to be any solid work on plugins for combining WordPress and OpenCart, I will tell you that it would be a major task.
Thanks,
- Brad
I too was trying to do this and ended up just linking the two seperate sites together. I did see the enclosed link just a few minutes ago. I have no idea if it works or not, but thought it was at least worth looking into. I believe it is just a matter of time before some hotshot genuis geek figures it out. It seems by the number of hits here that it is a popular topic.
Good luck.