question unity

Firstly take the Unity Answers tutorial, seriously

Slender

Slender was indeed crated with unity. you can even see the terrain and even some default textures on the ground!. I also think the is actually official knowledge. if you delve into Slender's files you will see that the file system is unmistakable. (its Unity's).

Whether slender used other software or not. (highly likely, since they would of coded slender behaviors etc)

import unity projects fps creator software

i have no idea what you mean here, im going to assume you mean this?

http://fps-creator.en.softonic.com/

if so then then answer is no unless you can export the level somehow. Unity is a much more advanced engine than this and certainly more powerful. there is no way you can import a unity level in Fps creator without stripping manually(your better off send the assets over and rebuilding it.

if you want to import models from FPScreator to Unity then it should work, you either need to find an export option in FPScreator or move the models over manually to your "assets" folder. Now this is for Models Only!!!!!!!!!!!!!! you will need to learn to code your own FPS system. you are obviously new to this. if you manage to import your models to unity, pressing play won't begin a playable level(it will just sit there). so you will need to setup your AI, Guns, Player, Camera etc. luckily Unity has inbuilt packages for that but you may want to tweak those. basically unlike FPScreator where the guns, AI, enemies are all set up for you automatically, Unity gives you the freedom of making it yourself.

in simpler words, your guns will no longer shoot, you/enemies will not longer move around if you import your level to unity. since your new , I would strongly recommend checking out some tutorials, even if your been using FPS creator for a while.

you can find tutorials at:

http://unitygems.com/

http://www.3dbuzz.com/

and good ol you tube.