I don’t understand your layout.
It seems like the fortigate is a LAN device like any other (with IP 3.3.3.254, belonging to the 3.3.3.0/24 network) that is “fed” from the mikrotik via a public IP 2.2.2.66 (but is it the fortigate ip or the mikrotik one?) the whole thing seems a loop, I had expected that you have two ways to the internet, one with gateway 1.1.1.105 and one with gateway 2.2.2.65.
BTW you probably have a typo, 1.1.1.104 is nowhere to be found in the layout.
Anyway, sure you can disable NAT for a given interface, but it depends on how you have setup NAT right now, if you used the usual Mikroitk categorization of interfaces as LAN and WAN you need to change that.
Best thing would be if you could review (don’t worry about the form, the paint sketch is good enough) the layout and description, and post an export of your Mikrotik “as is”, instructions here:
http://forum.mikrotik.com/t/forum-rules/173010/1