Tinderbox does not support direct import of RTF files.
RTF text can be pasted into note $Text, supported style and weblinks are retained. Use Paste-and-Match-Style (⌘+⌥+⇧+V) to paste a plain text version of the source text.
Styled text can be copied from $Text.
There is no bulk import method, i.e. for one or more discrete files. Note that bulk text import is supported for plain text.