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Yes you should be able to do that. I use LiFePO4 cells in all my Fenix and 4Sevens lights intended for use with primaries and have never had a problem with it. Edit: Note that the discharge curve of LiFe cells is quite flat and then it suddenly dies. They also have somewhat less capacity than a comparably sized LiCo cell. All of this just means that you will need to charge them a little more often than you would a LiCo cell. Enjoy you new light!
Note that you need an appropriate charger for them. Most Li-Ion chargers charge to 4.2 volts. LiFePO4 cells can only be charged up to 3.6 volts. Charging them higher than that will damage them, so you need a charger intended for them. If you do not already have one I suggest that you consider getting a hobby charger. See the link in my sig for more info on that if it interests you.