imo it´s much more easier than doing a real xml edit, but i´m also a bit confused, looking at the PISingFoil3.nif there is already an effect node 'EFFECT_gun' and an effect attached to this node in the kf (EFFECT:EFFECT_gun:EFFECT_INVASIONSHIP_LASERBEAM), so therefore with the 'PISingFoil.kfm' as animation (be aware, you need the nif called PISingFoil.nif in the folder where the kfm is) there should be an weapon effect.
in general the steps are quite easy, open the nif you want to add the effect and the corresponding kf, open the same files from an unit with the effect you want to use [called master]. Look in the txt keys form master kf how the effect node is called and in your nif if there is a node with this name. If not, look if you have already an effect node or not. If you have, rename this node to the name in the master kf. if not copy the node form the master nif to your nif. Now as last step add the effect txt key entry to your kf (if need raise the number of txt keys [if there is only are only two keys so far], copy the end txt key [now after adding new ones no longer the last] to the last txt key and add copy the text [also adjusting the time of the key]).