I want to draw my tiles and create my tileset on Pyxel Edit, draw background images like trees, bushes on adobe illustrator and add them to my tileset.

The problem is that i have no idea how to add these background images to my existing tileset, they become pixel arts when i import them to pyxel edit and i don't want that.

Is there any other program to create a tileset and import images to it?

I'm trying make a tileset like this one, some tiles are just pixels and other tiles look just fine:

Adobe illustrator is a vector tool. Vector images aren't stored in sprite sheets as the whole point of sprite sheets is to optimize sprites for game use. Vectors are as optimal as it gets.

To explain, pixels images are known as raster image. Vector images instead store vector points to draw the image. Vectors can be either 2D or 3D it doesn't matter.

Good engines like Unity and Unreal support SVG files. SVG files are vector formats like PNG and JPG are raster.

If you use a engine that doesn't support SVG you can use Blender (A powerful free 3D software that outperforms commercial software) to import the SVG and export as OBJ or whatever 3D vector format the engine can read.