I don't know of anything generic that can handle large data imports like this. Every CMS stores data differently. There is a (very useful) free add-on that can create new pages in bulk, but that won't import page data.

However, concrete5's API is very extensive and has all the functions to create new pages programmatically. It's probably about 4 lines of code to create a new page and another 4 to add a content block to that page.

You can also use the database API to connect to another database -http://www.concrete5.org/documentation/introduction/database-access...

So I'd suggest this would be the case of writing a tools script that loops through the pages in a ModX database and creates pages. For 1500 pages, you'd want to do this in batches, not try to do the whole import at once.

That's probably the easier bit, the harder parts would be translating a site map structure (not sure how it's stored in ModX), and deciding how to handle images and other more complex pieces of content.