1. That may be added.

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

• additude,
• additivies,
• aditives,
• adative,
• addetive,
• adictive,
• addicitve,
• addivtive,
• assitive,
• adpative,
• additiude,
• addective,
• additived,
• addaptive,
• adgetive,
• adjative,
• addictave,
• aditive,
• additin,
• meditive

• auditive.

The adjective form of the word "additive" is "additive".

The plural of the "additive" can be the "additives".

The singular of the "additive" can be the "additive".