You can import host functions and data to a WASM VM much like you would with, say, an embedded Python VM. The exact API depends on which VM implementation and host language you're using.
dlopen/dlsym support is still in the works (for example, see [0]). You can't call function pointers directly from WASM, naturally, but it is possible to wrap dlsym so you can use it from a WASM program. There just isn't a standardized/pre-made way to do it yet, so you'd have to write the glue code yourself.
like say you wanted to make a J2534 DLL http://www.drewtech.com/support/passthru.html
How would you export these DLL functions or even call functions from other DLLs (dlopen/dlsym, etc.)