Apparently not. Makes sense for the ones from Thermia Fractures, since you can only get them once, but the ones dropping from Ropalolyst should be tradeable.

Are amalgam mods tradeable?

Not tradeable. damage. Lifesteal does not apply to Two-Handed Nikanas.

Where can I farm amalgam mods?

The unique weapon Amalgam mods drop from the Ropalolyst. The non-unique ones (Serration, Barrel Diffusion, Shotgun Spazz, Organ Shatter) are awarded after sealing enough Thermia Fractures.