In case that you are looking for conversion of text files to Keynote, you have really only two options. Namely Keynote or cloud based iWorks suite.
Another type of txt to key conversion is related to Windows registry. It should be possible to manually create Windows registry keys (.key) in any text editor, provided you know what you are doing and know the syntax. This is usually done when you copy and paste some lines from internet to create your own .key file that can be used to add registry entries to your system.Open TXT file Open KEY file
A software multi-featured tabbed notebook