HMC does not provide an install source for the LPARs - it only manages the attached servers: create/alter LPARs, provide a console virtual terminal, update/upgrade server microcode (not OS!), perform call-home function in case of HW/SW problem...
For installation of OS on LPAR, you need to use whatever you choose as best solution. NIM is fine, but there's no reason not to install from CDROM or DVD e.g. if there's only a few partitions to do and you don't already have a NIM server.
For instance, if you have a tape drive or a DVD-RAM drive in the server, and you can DLPAR-move it around all your partitions, and then a mksysb or mkcd to physical media can help you along. If you want to automate the whole process, then I guess NIM is a prerequisite...
There are quite a few redbooks on LPAR systems - check 'em out first. Don't jump in at the deep end - I took an IBM course before I got my hands on the servers... Regatta systems first and then p5 systems.
HTH,
p5wizard