Open the problem in polygon, then in the lower right corner of the page there is link in the bottom of a rectangle it will be of the form : https://polygon.codeforces.com/p/[your handle]/[problem name], just copy it and paste it in the muashup and you are done.

If you want to add all the problems at once you should create a Gym contest instead, then follow the instructions to set up a contest from a Polygon contest descriptor. But I think you must have coach mode enabled or something similar for that. I'm not sure if there's another way to do that.

For those who will try to do it this days, just create empty mashup, and there it will be available to upload all problems from certain polygon contest.