Hi Felix,
Thank you for reaching out to us. Yes you can insert X page templates to Pro pages. Pro at the back-end uses Cornerstone so the template exported from X should work in Pro without an issue.
There could be a version issue, maybe the page template you’ve exported from X is using an old version of Cornerstone and where you’re trying to import has the latest version. This could cause compatibility issues. You can first update both X/Cornerstone and Pro to the latest versions and then try importing the template again.
After updating everything you can export / import a new test page and see it this works. I tested it on my local setup and it works fine. I’ve the latest versions of X/Cornerstone and Pro.
Let us know how this goes!