I have two unreal engine games in Linux, Everspace and LittleNigthmares
that dont recognize my Steelseries Stratus Duo but recognize
other more old gamepads of mine.

Is there a way to fix that.
For example some games have a gamecontrollerdb.txt file where you can
add your entries. I fix a couple of old games that way.

I've heard about ''importing'' the game in steam but i dont have steam.
But if that is a solution i guess an unreal game can also search for
gamepads id somewhere?

Usually games support xinput devices and if they are paid they will support others. Your best bet is controller emulation which is what steam does. There are other programs that do the same and that's all they do but I don't know if they have Linux versions.

Pinnacle is the one I use when needed but again.. don't know if they support Linux.

Just do a search for "controller emulation Linux" or buy an Xbox 360 or Xbox one controller. They are basically the standard and most popular controller's used on PC. They are also xinput so if a game says it supports controllers then there's a 99.9% chance they are talking about an Xbox controller.