The spells I make should be given to the player of my future mod somehow. kjeron mentioned the ADD_SPELL action. I don't know the syntax for it, but he said that Weidu inserts the spells renamed along the lines of SPPR710, with the first number being the spell's level and the other two chosen among available slots, 1 to 50. I have never and don't like the idea of fitting into any slots, but if this is the easiest way... For priests, certainly, the new spells should be available automatically on level-up. And even more important, wizards should be able to choose the new spells for the 1st level at character creation. The rest of the levels they will have to find on scrolls. My concern is having enough slots left over in the levels. Given all the mods people have made, is there space to stick a bunch of additions? And what are future modders going to do, if I fill up the space with my stuff? It's a poor system.
There is an alternative - the Add Innate Ability effect. Scripts could be written to put spells in the scroll of eligible priests automatically...