Are you sure your Paths in Illustrator are joined and not just grouped? Other than that, you can try to export your linework as a DWG from Illustrator if you haven't done that already.
For me works both to get a polyline from a "joined" path from Illustrator into Archicad:
1/ saving as PDF from AI, placing as drawing in AC and exploding
2/ exporting as DWG from AI and opening in Archicad
Depending on how complex is your linework, you can also just join lines into polylines in Archicad...