So "templum" is the nominative form? It looks like it should be accusative...
Templum is a neuter noun of the second declension. These end in -um in the nominative.
Note these two important rules about neuter nouns:
all neuter nouns have exactly the same form in nominative and accusative (No exceptions exist whatsoever)
all neuter nouns end in -a in the nominative and accusative plural. Again, no exceptions ever.
No, you can't combine nouns that way, like you can in English.
However, there are patterns for how to make a noun into an adjective. In this case it's templum --> templaris (Hence 'knights templar')
'He has a temple altar': aram templarem habet