No. You cant upgrade it at random. Mostly your netbook is integrated build in which you cant dissemble their component. Intel Atom is just designed for small laptop which contribute to small voltage with such a small performance capability.
Please don't modified netbook/laptop. It is designed that way by factory manufacturer. It is fix with the spec given. Altering it may deal many problem. You only can change Hard Disk,RAM, Internet Card. Other that these part, leave it behind.
The spec of Intel i5 is not compatible with the socket of your U100 MSI netbook and the power supply will not be able to cope the power supply that Intel i5 need. You only can change the RAM because the chipset of your U100 MSI netbook is supported up to 2GB. Make sure it is DDR2 RAM.
Other than that, don't touch any other component nor replace the CPU. Just touch RAM or if you want to upgrade the Hard Disk it should be fine.