Is it possible to make a desktop into a laptop?

To some degree, yes. It is possible in the sense that you can put a motherboard in a portable case, connect it to battery power, use smaller drives, etc... The main issues in doing so are heat dissipation and the power system / power management. Heat dissipation is the ability to remove heat from the system so that it does not overheat, a greater challenge in a small case. The problem with power is first that you will probably not be able find a portable power supply that can automatically switch from AC power to batteries. Even if you do, monitoring it may not be handled transparently by the operating system (ie a battery icon in the system tray), requiring additional software.