Well, you could always play as a FA to begin with. That's perhaps the most 'legitimate' way to going about that.
Sorry for stating the obvious, but what more is there to say?
Take a page out of the flares book:
"I met this Farmer once, he was in a really big battle so I came in to save him. I helped him fight off the Kusari law enforcement however he took serious injuries. Before he died, he told me I could have his Mamoru for being nice".
Or
"My friend gave it to me"
Or
"I killed a Farmer and took if off him"
All of those and many more in Spike Seadra's 'How to roleplay' for only $9.99!
In all serious, my suggestion is to go along the route of having a farmers alliance character. From there, play within the faction a bit and RP something like you didn't feel like the farmers alliance was worth being a part of or something. Of course, you'd have to write a bit of paragraphs if you want it to feel justified.