Illustrator to chemdraw
From p. 327 of ChemDraw 18 User Guide:


  Importing
  
  You can import graphics or documents from other applications into ChemDraw.
  
  Files
  Files that you insert become part of the drawing. You can edit a file only if it is in a ChemDraw format.
  To insert a file:
  
  
  Navigate to Edit → Insert File.
  In the Open dialog, select the file type from the drop-down list.
  Select the file and click Open. The file is embedded in the drawing.
  


Further, there is a table of file formats ChemDraw is capable of importing and exporting, from which it is evident that only the following graphical formats can be imported:


Bitmap (BMP)   
Graphic Image Format (GIF)  
JPEG (JPG, JPEG)  
Portable Network Graphics (PNG)  
Windows Metafile (EMF, WMF)  
Tagged Image File (TIF, TIFF)   


PDF or PostScript files can be attached as objects, but I suppose it's not what you are after.
Either way, it looks like there is no way to add AI files directly, so you need to convert them first to either raster (e.g. TIFF or PNG) or vector (WMF) format.
I agree with @Ivan that crafting structures in ChemDraw in the first place and then post-processing them with Adobe Illustrator is arguably a better workflow.