When you need to combine multiple 'simple' listings into a single 'variation' listing, here is what that process looks like.
NOTE: This can only be done thru non-Amazon channels.
Start by going to the channel's Listing page. Select the listings you want to merge by checking the small box to the left of the listings.
Once you have selected the listings you want to merge, click on the action drop-down menu and select Merge Listings.
Voila! Your simple listings are now combined into one variation listing.