The distinction Main/Lookup has only sense with incoming connections. From the outgoing connection scope, there's no difference between lookup or main: it's just a vectorized stream of data.
Pratically speaking, this means you can simply connect the main connection of your wanna-be-lookup tMap to the real-main tMap: since you cannot have more than one main connection, the additional one(s) will be automatically marked as lookup connection(s).
You can change the flow type for your incoming connections (from main to lookup and vice-versa) at any time doing a right-click on your connection, then "Change connection order"
It's quite easy to mess-up your mind while doing this. Just remember that your main connection is the one which starts with the component that has a green background.