It is actually possible to do.
first of all, create your spellbook ability(we'll call it "book"), set the <Base Order Id> to "absorb".
now create another spellbook ability(call it dummy or someting), also with the Base Order Id "absorb".
place the skill you want to add to the "book" inside "dummy".
At map initialization disable the ability "dummy" for all players.
After you've done this, do following to add the spell into "book".
Add the "dummy" ability to the unit you want to have the spell, and POOF! the spell is suddenly inside the "book".
Attached test map too.