Basically, you don't "make entities available from another game DLL", what you have to do is to code them yourself. That is, implement all the spawn/touch/think/etc. functions in your plugin, as well as the class export (LINK_ENTITY_TO_CLASS). That's what botman did. That's also what the guy who wrote HookMod (a plugin that adds a grapple hook like the one in Opposing Force to any mod).
When I say it's complicated...
