Hi,
I'm not sure if you can dynamically set the multi cell editor as the interfaces are different. Try this if it works, Use the Cell Variant for the column CL_SALV_WD_CV_STANDARD, and then set the Variant as Multi Cell Editor CL_SALV_WD_MULTI_CELL_EDITOR ( which has other Cell Editors ) to that.
You may refer to this document on dynamically setting cell editors to different rows : Dynamic Cell Editors in ALV
Regards,
Kiran