If I conquered before attaching Last Rites to a unit, can I use the effect of Last Rites if I attach it to a unit after the conquer?

No, you cannot use the effect of Last Rites if you attach it to a unit after you have already conquered the battlefield.

According to RiftJudge FAQ #9015 and RiftJudge FAQ #9486, the source of the ability (the unit equipped with Last Rites) must be present on the battlefield at the moment the conquer event occurs for the ability to trigger.

Reasoning:

1. Trigger Condition: The ability on Last Rites is a triggered ability that activates "When I conquer or hold."
2. Timing of Trigger: The trigger condition is met at the specific moment you establish control of a battlefield (the conquer event).
3. Requirement for Presence: For a triggered ability to be placed on the chain, the source of that ability must be on the battlefield to "see" the event occur. If Last Rites is not attached to a unit on that battlefield at the exact moment you conquer it, the ability does not trigger.
4. No Retroactive Triggering: Attaching the gear after the conquer event has already concluded does not retroactively trigger the ability, as the condition for the trigger (the act of conquering) has already passed.

In summary, because you had already conquered the battlefield before attaching the gear, the "When I conquer" condition was not met while the gear was present, so the ability will not trigger.