1. That may be added.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• edable,
• avadable,
• addorable,
• assable,
• adoable,
• audable,
• auidable,
• adouble,
• aduible,
• adroable,
• edidable,
• eddible,
• aveable,
• bidable,
• advable,
• afodable,
• eadable,
• aable,
• auddible,
• aviable

• ad blitz,
• addible.

The adjective form of the word "addable" is "additional".

The plural form of "addable" is "addables".

The singular form of the word "addable" is "addable". The word "addable" is already in its singular form, and it doesn't change when referring to a single object or person.