When inserting a Structural Member (in this example a DIN C150x51x12.2) from the Toolbox, you need to fill in the lenght of the part.

Unfortunately the 'Lenght' field doesn't accept calculations to be used. 

When you would like to extend the part with 100mm and you type 10.0+100, this results in a part-name with '+100' added and no extended length.

Is it possible to change the 'length' field to make calculations possible?