Yes, you can use assets you find outside of DiscMaster, but you won't qualify for the 100% DiscMaster assets prize category - and that's totally fine, that's an intentional challenge, and there are still two other categories: 50%+ DiscMaster assets, and using the theme of "media preservation"
You can also check the FAQ stickied at the top of this forum for more information about prize category eligbility. Hope that helps!