Hello Philippe,
Well you "have" TSM", since it's part of the CMOD license your company bought.
It's simply that you didn't install and use it.
But as Justin just said. TSM is the only solution for archiving with CMOD Multiplatform, except the cache of course!
The reason is that to achieve this high performance archiving/retrieval, you need to have a really close interface with the archiving/backup technology.
In that case TSM was a natural choice since both are IBM products.
Maybe NetBackup could be a choice, BUT I'm not sure it has all the features that CMOD needs, and creating an interface with it, would mean probably requesting lots of features to NetBackup... maybe some would be not "compliant" with the direction of the NetBackup product... I don't know.
In all cases, you could install TSM in the same server as your CMOD server (not always recommended, but there is nothing technically to prevent you to do it).
Cheers,
Alessandro