It can be a problem. RES not adapted (born and raised there over several generations) to cold winters can die. That approach (just put some in and see if they live) to stocking is stupid in most cases. It leaves you with a gap in size in that species which makes it much harder (and more expensive) to fix. Plus if you are counting on the species to fill a biological niche and it won't work you have to change the plan on the fly with no guarantee you can fix it.