Quick tip: Here's how I successfully created a combined feed from my YouTube subscriptions.
YouTube does not seem to offer a direct export of your subscription feeds as an OPML file that you could import somewhere else, but it does provide RSS feeds for tags and users. See YouTube's RSS feed information.
Here's how you can create a single feed out of multiple tag and user feeds:
feed://www.youtube.com/rss/tag/yourtag.rss
Note: In the above example, substitute your desired tag for "yourtag".
See my example YouTube Channel page to get a feel for what it looks like.
If you like this tip or have ways to improve on it, leave a comment to let me know.