I guess what you are calling coefficient of water pressure is nothing but the water pressure in a column considering losses due to seepage. In SLIDE you are able to run a groundwater analyses (FEM) that will compute these losses and then a slope stability analysis (LEM) may be run that considers the results from the FEM groundwater analysis. So, if you are going to consider seepage head losses you will need to compute them first. Otherwise, you may be conservative and input a groundwater level only.
Nevertheless, if you are not alowing or should not allow plastic deformations in your model you do not need to perform FEM analyses.
Hope this helps.