O.o wow lol, i don't remember that, though i have to say, i very RARELY go back to eAthena forums for information, so that could be why i don't remember this.
Also for adding a skill to the skill tree it'd be like this:
/* Note - Only 84 skills are allowed per class (unless you edit the src)
* JobID, SkillID, MaxLv{,JobLv},Pre-Req_id_1,Pre-req_lv_1,...,pre-req_id_5,Pre-req_lv_5 */
//Stalker
4018,1,9,0,0,0,0,0,0,0,0,0,0 //NV_BASIC#Basic Skill#
4018,142,1,0,0,0,0,0,0,0,0,0,0 //NV_FIRSTAID#First Aid#
4018,48,10,0,0,0,0,0,0,0,0,0,0 //TF_DOUBLE#Double Attack#
4018,49,10,0,0,0,0,0,0,0,0,0,0 //TF_MISS#Improve Dodge#
4018,50,10,0,0,0,0,0,0,0,0,0,0 //TF_STEAL#Steal#
4018,51,10,50,5,0,0,0,0,0,0,0,0 //TF_HIDING#Hiding#
4018,52,10,0,0,0,0,0,0,0,0,0,0 //TF_POISON#Envenom#
4018,53,1,52,3,0,0,0,0,0,0,0,0 //TF_DETOXIFY#Detoxify#
4018,149,1,0,0,0,0,0,0,0,0,0,0 //TF_SPRINKLESAND#Throw Sand#
4018,150,1,0,0,0,0,0,0,0,0,0,0 //TF_BACKSLIDING#Back Sliding#
4018,151,1,0,0,0,0,0,0,0,0,0,0 //TF_PICKSTONE#Find Stone#
4018,152,1,0,0,0,0,0,0,0,0,0,0 //TF_THROWSTONE#Stone Fling#
4018,2,10,0,0,0,0,0,0,0,0,0,0 //SM_SWORD#Sword Mastery#
4018,44,10,0,0,0,0,0,0,0,0,0,0 //AC_VULTURE#Vulture's Eye#
4018,46,10,44,10,0,0,0,0,0,0,0,0 //AC_DOUBLE#Double Strafe#
4018,124,1,46,5,0,0,0,0,0,0,0,0 //HT_REMOVETRAP#Remove Trap#
4018,210,10,50,1,0,0,0,0,0,0,0,0 //RG_SNATCHER#Gank#
4018,211,10,210,4,0,0,0,0,0,0,0,0 //RG_STEALCOIN#Mug#
4018,212,10,211,4,0,0,0,0,0,0,0,0 //RG_BACKSTAP#Back Stab#
4018,213,5,51,1,0,0,0,0,0,0,0,0 //RG_TUNNELDRIVE#Stalk#
4018,214,5,212,2,213,2,0,0,0,0,0,0 //RG_RAID#Sightless Raid#
4018,215,5,217,5,0,0,0,0,0,0,0,0 //RG_STRIPWEAPON#Divest Weapon#
4018,216,5,218,5,0,0,0,0,0,0,0,0 //RG_STRIPSHIELD#Divest Shield#
4018,217,5,216,5,0,0,0,0,0,0,0,0 //RG_STRIPARMOR#Divest Armor#
4018,218,5,211,2,0,0,0,0,0,0,0,0 //RG_STRIPHELM#Divest Helm#
4018,219,5,212,4,214,5,0,0,0,0,0,0 //RG_INTIMIDATE#Snatch#
4018,220,1,221,5,0,0,0,0,0,0,0,0 //RG_GRAFFITI#Scribble#
4018,221,5,222,1,0,0,0,0,0,0,0,0 //RG_FLAGGRAFFITI#Piece#
4018,222,1,223,1,0,0,0,0,0,0,0,0 //RG_CLEANER#Remover#
4018,223,1,216,3,0,0,0,0,0,0,0,0 //RG_GANGSTER#Slyness#
4018,224,5,223,1,0,0,0,0,0,0,0,0 //RG_COMPULSION#Haggle#
4018,225,10,219,5,0,0,0,0,0,0,0,0 //RG_PLAGIARISM#Intimidate#
4018,1005,1,0,0,0,0,0,0,0,0,0,0 //RG_CLOSECONFINE#Close Confine#
4018,389,5,51,5,213,3,0,0,0,0,0,0 //ST_CHASEWALK#Stealth#
4018,390,5,0,0,0,0,0,0,0,0,0,0 //ST_REJECTSWORD#Counter Instinct#
4018,475,1,225,10,0,0,0,0,0,0,0,0 //ST_PRESERVE#Preserve#
4018,476,5,215,5,216,5,217,5,218,5,0,0 //ST_FULLSTRIP#Divest All#
4018,410,1,0,0,0,0,0,0,0,0,0,0 //WE_CALLBABY#Call Baby#
4018,681,1,0,0,0,0,0,0,0,0,0,0 //ALL_INCCARRY#Enlarge Weight Limit R#
//Custom Skills for skill tree
4018,89,10,1,10,0,0,0,0,0,0,0,0 //WZ_STORMGUST#Storm Gust#
//Add more as needed.
/* Note2 - You needed to add them to each class you want able to use the NPC. */
The above, is a list of the stalker's skill tree according to: rAthena/db/(Pre-RE | RE)/skill_tree.txt
All i did was add, StormGust to it's skill tree, with an impossible requirement (Basic Skill Level 10). This way they can't use skill points to level it up. BUT you can still give them the skill using an NPC.
As for your NPC, the way it is, should work fine, if all you did was add skills to their skill tree.