I'm not a builder and only have DIY experience, but what I'd do is to make the break now, before the new concrete is laid.
In other words, using a 230mm cutter, perhaps backed up with a breaker/chisel etc, separate the two parts of the slab.
It only needs to be a separation of a couple of inches wide. Just so that any work on the external slab in the future does not affect the internal part.
You say that you're 'in the process'. Does that mean that you're about to, or that you have started.
If you haven't started yet, the imprinted concrete guys will probably separate the slabs for you before they start.
But I'd use a different builder for the separation, just to make sure it's been done properly. Depends on how well you can trust the imprinting concrete contractor.
I'm surprised how you managed to have a building on a previous slab that is part internal & part external. but I guess that's another story.
I suggest you wait for confirmation or other suggestions from the more experienced guys on the forums, but put the imprinted concrete on hold until you're satisfied with what you're doing.