Hey there. The weapon system included is setup to use skeletal meshes as this allows you to use sockets and such for things like muzzle flashes, projectile spawn points etc. In order to change it to static meshes, you would need to go through and change the system quite a lot on your own. In general, using static meshes for weapons is not advisable as static meshes are really meant much for environmental props and such.
You can open up your static meshes in a 3d modelling software, such as blender (which is entirely free) and add a single bone to them to then allow an easy conversion to skeletal mesh in UE4, then allowing you to use the weapons as a skeletal mesh, using sockets, animations, etc.
Hope that helps 