If you for whatever reasons need to convert your text files to some very old formats, such as .rft files (Revisable Form Text) for IBM DisplayWrite, you will need to use some of the dedicated conversion utilities, such as ABC Amber Text Converter. This is mainly because .rtf files are old and rarely supported in modern text editors or word processors.
However, you might have been looking for a way to create RTF (Rich Text Documents) from text files, which is covered under txt to rtf conversion entry.