Yes, you can definitely use your own 1st and 2nd stages with our FFMs. We have a mask called the Neptune II. This mask looks very similar to the Neptune Space mask, except it has no integrated 2nd stage. It comes with an adapter to attach your own 2nd stage straight to the front of the mask.
Another way to integrate your existing setup with our Neptune Space mask is by using your G250v as an octo. The Neptune Space already has a 2nd built into the mask, therefore, you could either have your G250v attached as a normal octo or you can buy and adapter which would attached you octo to the divers right side port. For the last configuration, this would allow you in case of an emergency to not be forced to remove your mask from your face.
Either way you go, using your own 1st stage is common and is not an issue.