On the DesignSpark forum many people are asking why a STEP file is locked and can't be updated when imported in DSMech.
And they are right to think that this is a very bad point !
I completely agree and I hope the developers will allow STEP modifications in future versions. DSMech will gain popularity if they do so !
But there's a solution and it's called SketchUp...
: there's a much easier way as mentioned by Didier in the comments. Thanks Didier !