Although I am no expert, two things spring to mind:
1. 15 metres from the top of a cupboard to the bottom is a VERY big cupboard! Are you sure you have the right dimensions?
2. Reducing the line by 15m will have very little positive effect on the attenuation. If you need to make up the difference with a separate piece of cable, you would probably make things worse (by introducing another connection).
In short, don't do it!