Yes you can add your past work on Upwork as well under your portfolio section. This will help clients to view your experience and past work that you have already done. If you have any additional questions feel free to post them here, thank you.
Hi, yes, I have some t-shirts design and some older flyers, I want to place them on a free and cc0 blank mock up(a white t-shirt with a smart object to place the design on it and a blank paper with some shadows), to make them look more beautiful in the portfolio, is it allowed or I have to make the mock ups by myself?
Yes, Use mockups! But make sure the work is yours. I myself view alot of portfolios that have stolen art or mockups that the images and watermark weren't even removed.
I use them and have several in my portfolio. Clients want to see the work in the real world and mockups give us that option. Plus, I've sent mockups to protential clients and got hired.
GO FOR IT!