You can definately do this, the only missing part is the checksum calculation. Some options:

https://github.com/massmux/HowtoCalculateBip39Mnemonic

https://github.com/bitaps-com/mnemonic-offline-tool

and above mentioned coleman.

And of course, a SeedSigner can also do this. It will adjust the final word to a valid checksum (IIRC).

Please use these safely, e.g. offline when doing this for reals.