Scientists at the Scripps Research Institute have already created organisms with synthetic DNA letters, but they weren't ready for the real world when they couldn't even keep the artificial base pair in their genetic code. However, the team has made a lot of progress since then: they've produced the first stable semi-synthetic organism. The bacteria now holds on to its human-created X and Y bases while it grows and divides, much like the natural A, C, G and T bases. The key, researchers say, was to tweak existing techniques.
At first, they fine-tuned a nucleotide transporter (which carries the materials needed to copy artificial base pairs across the cell membrane) so that it wouldn't make the bacteria "sick" while using the new letters. They also made their Y letter easier to copy. And as a sort of insurance, the scientists used CRISPR gene editing to make their organism reject genetic sequences that don't have X and Y -- it's effectively impossible to lose the synthetic data.
You're still a long, long way from seeing this hybrid life used for practical purposes. It's only useful in single-celled organisms, and you can only make it store gene info. It's another matter entirely to get this data into RNA, let alone to produce complex organisms. So long as there's progress, though, Scripps sees a bright future ahead. Eventually, you could use these letters to create new functions that help with discovering new medicines and "much more."