- In Indesign save as a pdf
- then open the pdf in Illustrator
- then save as an .ai file.
This seems to work fine, the art board is the right size and the text (kind of) editable.
BUT, sometimes Illustrator throws a wobbly and substitutes the fonts for no good reason.
In this case you should:
- export as an .eps in Indesign (PostScript Level 3, embed all fonts)
- open the .eps in Illustrator
- save as .ai file
This has the disadvantage that Illustrator won’t create the right size art board (it will be the default A4 size) and the fonts are outlined, but at least it looks right.