Just wondering if it's possible to add bump and spec layers to freeware aircraft such as Posky and TDS. Without them, the aircraft looks very matte, flat and boring. 

Planes who are not designed for bump and spec cannot be upgraded with them by users.

The model materials have to be set up to use bump and spec files. If it's not, I've heard some repainters have been able to alter the model materials attributes using ModelConverterX. I've never tried rhis myself, so I can't provide any further insight. Be aware that recompiling a model may be in violation of licensing agreements.

MOdel Converter can be used to convert FSX models to X-Plane models.
I do not think you can manipulate existing aircraft for FSX.

It violation of rights anyways.

https://www.scenerydesign.org/modelconverterx/

"But besides converting between formats, the tool has become more versatile and can now also be used as an object viewer and object editor. Some of the editor functions are:

• Add special effects

• Edit material settings..."

As for a violation, that would depend on the terms of the EULA accompanying the model in question. As you know, in some cases, modifications MAY be allowed for personal use, credit to the original author or non-commercial purposes. Also, an exception may be granted by the author.

Thanks for the info