Is it possible to modify/extend the sopimport and lopimport nodes for custom in-house schemas?

No, not currently. It has been requested before, but it isn't currently in the plans for the next release. Our recommendation currently has to be that you need to create a custom import/export loop for your custom primitive types.

That's what we're doing currently (custom logic), just wanted to check that there wasn't already something built-in, thanks!