I can confirm that the SnapMap builder does work with external projects. I’ve got it working with multiple layout types including several of the packs from marketplace. It is a bit finicky though. The easiest thing to test on your maps is to increase the error tolerance to a much higher number and then see if it builds the map, it may have overlapping geometry but it should help show where the problem is.
Another problem I ran into a lot at first was the ordering that it builds. At first I was just adding nodes and attaching and not paying attention to the order and that can cause problems.
The other thing I have found to help when building them is to start simple and then add one room at a time in the flowchart until you find the error.