biblatex's data model is only extensible if you use Biber. If you stick to BibTeX as backend, you cannot add new fields. Sorry.
You will have to get backend=biber, running. With a not completely outdated system that should be as easy as changing backend=bibtex, into backend=biber, and telling your editor to use Biber instead of BibTeX: Biblatex with Biber: Configuring my editor to avoid undefined citations.
The versions of biblatex and Biber need to match. Normally that should be the case, but if you installed the two packages at different times and did not update your system when you installed the packages, you might get version mismatches. So running an update (if available) is recommended.
For the most part Biber parses .bib files in exactly the same way as BibTeX (there are a few small differences in how the two handle comments), so you should normally not get any issues when switching from BibTeX to Biber (unless your .bib files were very weird to begin with).