Let's Encrypt actively forbids using the same keypair as an account key and as a subject key in a certificate, so you won't be able to do that. But if you've pre-generated a particular key that you happen to want to serve as your account key, that wouldn't be forbidden by policy.
I don't know for sure whether any current client supports importing rather than generating an account key, so I anticipate you'll need to dig into the code of current clients and potentially modify it to do what you want.
I guess that the ZeroSSL and gethttpsforfree web clients do require users to deal with account keys explicitly, and there's le.pl which is the basis for ZeroSSL, so that might be one option.