A couple things...
- You can’t retroactively encrypt a Time Machine backup especially without the source available
- Time Machine is not designed to be an archival system
Time Machine is a back up tool meaning it examines a filesystem on a a computer and makes copies of files based on certain criteria. In this case, it copies over only files that have changed since the last back up. Additionally, it will back up system configuration files specific to your system. Without the source, Time Machine simply can’t function
As for archiving, you save data, not computer settings. The plist for your program (say Microsoft Word, for example) is useless in the future. Even the software that gets backed up will be OS and system specific. There are apps that worked fine just one or two versions of macOS ago, but now with Catalina, they don’t work. Without the correct system (hardware and and OS) these settings can’t do you any good.
Going forward, what you can do to archive things on an encrypted volume is manually traverse the folders on the Time Machine backup and copy them. The folder structure is the same as what’s on your drive (i.e. /Users/user/Documents), the only thing is that you’ll have multiple copies for each date Time Machine ran. It will be time consuming, but it will achieve what you’re looking to do.
Going even further Forward, it’s worth thinking about how you Structure your files for backup and archival now. This will make it easier to store your files for archival purposes without having to negotiate the complexities of Time Machine.