So canon says Thror, Gandalf and Aragorn entered Moria. Except for Aragorn, we also know WHY they did this.
Somehow Gollum got in as well, he was there just before the Fellowship arrived.
BUT Had Gollum ever entered Moria before?
Curious old Smeagol? Nosy old Smeagol? YOU BET. (I would further bet, that he went in and came out by the Dimrill Gate entrance on the east: EXCEPT finding a different way in/out would be more interesting, and would be much more flavourful for a source book. )
So why does a Ranger enter Moria?
Not for gold. Not for Revenge. (Though these were good enough purposes for Thorin and Co.)
It is fair to assume that Aragorn, just like Gandalf, was looking for someone.
A quick answer would be his Hunt for Gollum. But a more intriguing pursuit might connect with others who have entered Moria and then Aragorn's excursion can be placed at any time in his past.
The absolute Dread of Moria should displace anyone from settling a new colony there. I don't like that as a motivation, it makes no Tolkien sense (unless you are Balin).
So it's (a) Cash/ Treasure/ Mithril. (b) Revenge/Pursuit (c) Rescue / Tracking (d) Curiosity (e) Accidentally took the wrong turn...
And which of these might make a Campaign to fit a source book?