In PathFinder, click on the part(s) you want to place in the new subassembly.
Note:
To select more than one element, hold the Ctrl key and click each element.
Choose Home tab→Modify group→Transfer 
.
On the Transfer to Assembly Level dialog box, click on the assembly where you want to create the new subassembly.
On the Transfer to Assembly Level dialog box, click the New Subassembly button.
On the Create New Subassembly dialog box, in the Template box, select a file to use as a template for the new subassembly.
In the New File Name box, type a new name for the subassembly.
In the New File Location box, type a new file location for the subassembly.
Specify how you want to position the selected part(s) in the new subassembly. For more information on specifying the position, see Create New Subassembly Dialog Box.
On the Create New Subassembly dialog box, click OK.
On the Transfer to Assembly Level dialog box, click OK.