Hi TheLeaves,
I just answered your other related post over at https://www.method.me/cs/forums/p/9480/31917.aspx
There arent any restrictions in place as to which fields can be used as Merge. Any table/field you have access to in Method can be used in some fashion!
I could really go into great detail here, as there are some caveats to this, and it really depends on where and how you are trying to grab these merge Fields. Im guessing your other post was more specific to what you were looking for, so I posted the answer over there.
Thanks for Posting!
Ben