I fgiured out how to replace videos. It is very simple.
First, empty FEC file is only 15 Bytes, 12-th Byte define how many videos will be played in game (max 12 videos).
Secondly, before every movie there are some Bytes but I don't know what is their meaning (first Byte in these Bytes = number of movie, but I noticed it can be random?).
Thirdly, every movie is a .smk movie (magic numbers SMK2 - 53 4D 4B 32) and it is possible to replace but you have to use old RADTools app to convert .avi to .smk. Videos in game can have max res 640x480.
So, in every Monolith game where there is FEC file it shoud be possible to replace movies!