I just have a spider as a static mesh, but I want to make it as a skeletal mesh…so I can replace my enemy mesh, which is a skeletal mesh with the spider. But every time I try to replace the enemy mesh, it shows me just other skeletal meshes, nut not my static mesh… so can I convert my static mesh in…

You have to export your static mesh as skeletal mesh in your 3D application. There is no function that can change a Static Mesh into a Skeletal Mesh. <3