Spellbooks

Hi,
Im about to create a Unit which is able to Sell other Units.
The Problem is that it shall be able to sell about 15 other Units - and the Unit only got 12 free spaces. Normally i would use the "spellbook" ability now, but this is only for spells, not for selling Units.
So, is there a way to make my Unit be able to sell more then 12 Units?
Like, a button (ability?) and when you use it, your redirected into another menu where i can store sell-able units...

There are multiple possible solutions for this:

• Have multiple units that sell units.
• Have one unit that sells units which you can morph into another.
• Use spellbooks and the Charge Gold&Lumber ability, then use triggers to create the units when the ability is cast.