For industrial applications, importing element matrices from the various codes is often the solution that best answers the need to be able to go back to these codes once the SDT has been used to solve a particular problem. You can use FEMLink for that purpose.
Can you easily add new elements ?
elem0 gives an open specification of what element functions must do. You can thus extremely easily add new elements that will be compatible with all function (model building, plotting, mesh generation/handling).
Examples of elements built by various users are a solid element for fluid diffusion in soils, a version of quad4 for composite shells, a version of hexa8 for piezoelectric materials, ...