add image to a post with just code

case: I am syncing products from a external xml feed into wordpress. The xml feed also holds image urls which i want to copy as featured image with each post that is saved to WP.

So, i could just write some functions to do that job for me, the uploading into the wp-upload folder and there might be some building wp functions to connect the image to the post, so i can get it working but i wonder if i am not just overlooking a great build in wp function that can do the same?

Anyone did this, and what was your approach?