I purchased pixel composer from this itch.io site.

In the past you have said that when the Steam version is released, you will provide keys for the Steam version to people who bought it here.

I am not comfortable using this itch.io site to download the latest version.

That's why is it possible for you to issue me a key?

Every itch purchase already comes with a Steam key. You can redeem the Steam key on the download page. More information can be see here https://itch.io/t/67604/how-to-get-a-steam-key-from-your-itchio-purchase