I really enjoy modifying the rules and creating content (such as new classes) in PnP. One of the classes I have always felt was missing from the standard roster of classes was an arcane warrior, somewhat analogous to a Paladin as a divine warrior or ranger as a nature warrior. Consequently, I would like to create a Mage Knight class, that has casting as a sorcerer (but only going up to level 4 spells by level 20), but a full BAB like a fighter, barbarian, ranger or paladin. If it proves easy enough, I could throw in a couple of other abilities for extra fun (after all, the paladin and anger also get other stuff, ranging from lay on hands to trackless step and so on).

Is it possible to start from one class as a template and modify it to create an entirely new class? If so, could somebody point me to a step by step guide for newbies as to how this can be done? Will the new Mage Knight class be available as a choice in the campaigns that come included with the Neverwinter Nights Diamond Edition?

If this works, I will create the class and play it in one of the campaigns (probably the OC, leaving me to try the Shadows of Undertide and the Hordes of the Underdark, which I understand from other posts are connected story-wise, with my hopefully viable Sorcerer-Druid-Shifter build).

It's an interesting thought, but I'm not sure why you'd want to do this; You can easily build a multiclass character with 16+ BAB and up to lvl 8 spells. (Lvl 2 Fighter, Lvl 8 Wizard/Sorcerer, Lvl 10 Eldritch Knight, for example-this can be weighted more heavily towards feats by increasing fighter levels instead)

The only way I can see this being worth the effort (aka, filling a character type that the game currently doesn't have) is if you did the following to essentially make a variant spellsword base class;

REQUIRED (in my opinion);

A) provide increasing reductions to spell failure chance as the class progresses-by the time it hits level 10, it should be able to wear Mithral Full Plate with no spell failure chance (this is an inherent class feature, like rage)

B) Must have High BAB, and probably both a high Fort and a high Will

C) HD must be D8, maybe D10

D) Either 4 or 6 skill points per level, with Spellcraft, Tumble, Concentration, and one of the speech skills as class skills (plus a few others)

E) Should have same weapon and armor proficiencies as a fighter (maybe barbarian/cleric/ranger if you are desperate to downgrade it)

F) Spells are obviously required, but it would need to be done one of two ways; either maxes out at lvl 4 with more slots per spell level than any other class, or maxes at 6 with a regular number of slots. Spell list would consist of damage and buff spells, with maybe a couple of CC's like grease