I'm currently working on a homebrew subclass for wizard, this subclass' biggest feature would be the ability to learn spells from lists other than the wizard spell list. As an example, taking this subclass would allow a wizard to copy the cleric spell bless from a scroll into her own spellbook. Currently I can't figure out a way to make this work in Beyond, and I'm not sure if it's just not possible or if I'm doing something wrong.

For balance purposes this ability is granted at 6th level rather than 2nd, so I can't just add the addition lists to the subclass itself, I would want it to be gained as a class feature

Under Basic Information, add the cleric spell list as an Additional Spell List.

That would grant the spell list immediately upon taking the subclass, is there a way to do this that will only activate at 6th level?

Quote from IamSposta >>

Under Basic Information, add the cleric spell list as an Additional Spell List.

 Even after doing this it doesn't allow me to add spells from other classes to my spellbook, it only allows me to pick one spell from each class to learn as soon as I hit 2nd level

Quote from DrewconicSorcerer >>

That would grant the spell list immediately upon taking the subclass, is there a way to do this that will only activate at 6th level?

 No

Quote from DrewconicSorcerer >>

Quote from IamSposta >>

Under Basic Information, add the cleric spell list as an Additional Spell List.

 Even after doing this it doesn't allow me to add spells from other classes to my spellbook, it only allows me to pick one spell from each class to learn as soon as I hit 2nd level

It shouldn’t, you must have done something hinky.

Here:

Quote from IamSposta >>

Quote from DrewconicSorcerer >>

That would grant the spell list immediately upon taking the subclass, is there a way to do this that will only activate at 6th level?

 No

Quote from DrewconicSorcerer >>

Quote from IamSposta >>

Under Basic Information, add the cleric spell list as an Additional Spell List.

 Even after doing this it doesn't allow me to add spells from other classes to my spellbook, it only allows me to pick one spell from each class to learn as soon as I hit 2nd level

It shouldn’t, you must have done something hinky.

Here:

 Whenever I do this is just give me the option to pick one spell from the list, I'm not sure why either, I didnt change anything else

That’s because you’re doing it in the wrong place. NOT as part of any class feature, under “Basic Information” for the whole subclass. And not “additional specific spells,” under “additional spell list.”