If you simply want to add extra information about units that AMO has already added to the db, the easiest way is to use mif files to add the data to the inventory section for the unit. I have found that using the alternative method (using templates) to store the data for the unit is VERY frustrating, as often the data is overwritten.
My understanding is this - if you place information in a mif file in the clientws folder on an agent machine, the agent will collect it and send to the database. If the information is NOT structured in such a way to match a pre-defined template, the data will simply be added to the Invenotry tree for the unit.
So, in the example below, if you put a .mif file (with any name) with the following contents in clientws to be collected by the agent, it will create a section under Inventory called Information, with a branch called General2, with an attribute called test2 with a value of test.
This way, you simply have to have a way to get the data out of the other system and then use a script to create the mif files.
crofty
Start Component
Name = "Information"
Description = " "
Start Group
Name = "General2"
ID = 2
KEY=2
Class = ""
Description = ""
Start Attribute
Name = "test2"
ID = 3
Description = ""
Type = String(64)
Value = "test"
End Attribute
End Group
End Component