If you have created a multi-body part, you have the option to split up the bodies into separate parts and place them into an assembly. Essentially after creating the multi-body part, go to Insert > Features > Split. All bodies will be listed. You can double click the bodies to give them part file names.
Pressing OK will save all the bodies to part files. Right-click on the Split feature and choose "Create Assembly". On the next property manager page press the browse button to determine assembly name and location. Pressing the green checkmark for an assembly of the saved part bodies will place it in a fixed space in their original positions.