Tinderbox does not support direct import/export of RTF files.
RTF text can be pasted into note $Text.
Styled text can be copied from $Text.
There is no bulk import method, i.e. for for one or more discrete files. Note that bulk text import is supported for plain text.
Styled export to file is possible via HTML export and most RTF-capable editors will import well-styled HTML (styled using CSS). There are some limits to which text styling features can be exported via HTML export.