G7, the old TR001 etc are colorimetric definitions of output with specifics for the printer to actually achieve. IF (big if but absolutely possible) a print house conforms to such standards, all one needs is to find the associated ICC profile for conversions to CMYK and soft proofing. Just as if you owned say an Epson 3880 and wanted to print on Exhibition Fiber paper, you could download Epson's profile for that printer and paper and off you go. Yes, not all ICC profiles (even from Epson) are equal due to attributes such as the software used to build the profile and many other factors. But the point is this: you need an ICC profile that correctly defines the output for soft proofing and conversions. If you don't have such a profile, don't consider soft proofing with a profile that doesn't define the output and absolutely do not convert that data to that CMYK output color space. It's that simple.
If you can't do this, you can't cross render to proof the data. You don't have the recipe for CMYK!
So for the OP, you need to contact the printer and ask about an output profile. They may provide one. They may state their output conforms to an output standard like G7. That may or may not be true but it's a good start (and there are ways to confirm if this is true or not).
Lastly, you really want the print shop to provide contract proofs! And if possible, you want to take those contract proofs to the press run!