As I found out, there seems to be no way to apply new armours to the game unless you use FT Improver.exe to play the game. You can of course simply REPLACE a .spr of the original game, let's say power armor original game sprite with t51b Power Armor. If your goal is to ADD a new type of armor then you must do the following :
1. Install FT Improver to your Fallout tactics folder.
2. Open Improver_data/ArmourTypes.txt
3. Add a line at the bottom that looks like this : {PowerT51b;Pow;0;0;0} ("Pow" is the sound of the armor when moving with it)
This will create a new type of armour titled "PowerT51b" for male AND female (same sprite for both).
The name of the sprite file will have to be the same (PowerT51b.spr) and will have to be located in core/sprites/characters.
Last steps :
1. Open Ft Improver.exe and then Entities editor
2. Open original Power Armor .ent file
3. Save it using another name
4. Under Armor option, choose your new armor name
5. Open Level editor and place your new armor somehere in a map
Using FT Improver.exe to play the game, your new armour should work perfectly!!