that would rather defeat the point, since fish breaks compatibility by design, that's part of what i like about it.
i can however appreciate that breaking bash compatibility is not for everyone.
i mix use of bash and fish. but more often than not i find myself in a situation where i switch to fish because it makes some complex command easier, than the reverse.
There are a few plugins you can use with oh-my-fish and similar tools, for example `fish-foreign-env`, which provide a command (`fenv`, in this case) which runs a given command in bash and then transfers environment variable changes to the fish environment from which it was called.
It's not bash compatibility, but it is good enough for one-offs and even for many scripts and tools that expect to be sourced in your dotfiles from bash.