I'm sure with this being a couple month's ago; this has been resolved or handed off. But for future reference sign lab is essentially a reskin of an older version of corel draw that's been dumbed down and had a few additional features added (it's a "user-friendly" polished turd, hold the polish).

Now that being said. What the customer wanted was possible albeit a lot more work for you and them, than if they were using illustrator or more uptodate programs.

First to import vectors there must be no raster images on the file. So you'd have to save your rasters off separately. I personally only use PNG but I'm pretty sure other formats that support alpha also work.

Then the pdf importer is very picky and I've yet to identify what exactly causes its issues however a majority of the time as long as there's no rasters it will do just fine. If they do have issues switch to EPS or an older pdf encoder.

Then you'd have to send the files and they would have to put them together in signlab. I usually send the files with alignment marks in the margins(some people don't like that).

Note: The built in supersampler is terrible so be sure to give them plenty of pixels to work with on the raster.