Martine's wonderful tutorial on merging package files makes it all so simple.
Patience is required. It can take a long time to merge a lot of files. Also the "Save As..." function can take a long time.
Is there any limit to how many can be merged togather? SimPE seems to fall over if the file is too large, on Saving. Sandy says 200 Mo is the maximal size I give to my merged packages.
Some package file hairs can be merged, but some older hairs resist merging and throw an error in S3PE.
(Older) Patterns should be installed as Sims3Packs only.
Counters should be installed as Sims3Packs or may develop problems with cornering.
http://martinesimblr.tumblr.com/cctutorial link to Martine's updated material