Hello everybody

I like to use a regular 3D sketch as a layout at the assembly level, and control almost all main important dimensions of my parts through it, and that has been so beneficial to me so far.
The problem is that sometimes I need to modify assemblies of other's, and those, of-course, have their parts and sub assemblies already inserted into the general assembly and when I try to add a 3D sketch so that I can re-link main important dimensions to it, I can not drag it to be at the top of the assembly before all parts.
any Ideas on how to drag it there, or why it is not allowed?

I would highly appreciate help.

Thank you very much in advance