Neil Hodgkinson (MSFT - PFE)
Probably the simplest way to do this is to isolate the two site collections you want to migrate into their own database. You can do this using stsadm.exe but if you need more info I'll provide it.
Once you have the sites in their own database you can then PRESCAN the database and assuming no errors the next thing to do is to backup the database and restore it to the MOSS farm sql server. Onc you have it here you can use a contentdbattach process in stsadm or from the central admin > application management > content database
This process will upgrade the database and sites within it to MOSS.
Hope that helps