Since here is no feature like extrude boss or loft boss while in assembly, Is there any way to copy a 3D sketch made in an assembly file to a blank part template so that I can extrude that shape then put it back into the assembly and mate it to my other components.
I have an alternate solution, which may or may not be helpful.
Create a new file and save it. Insert this file into your assembly.
Within the Assembly Mode, use the Edit Component feature and use can edit the new file with the Assembly reference and create the required 3D Sketch in the new part.
I agree with SolidTweaks, but you don't even have to create a part, save it, then insert it into the assembly.
In the assembly, just choose Insert - New Part.
A new part is added to the assembly, and you are put into edit part mode. Now you can use any of the part based commands you are looking for. This is the basis of Top Down Design, so check that topic out in the help file and tutorials to get a feel for how relationships, mates, and constraints are going to be affected.
Of course. My bad. :D
try "save as" Part file.