I've seen a few modders look into this over the months and to the best of my knowledge nobody has cracked that nut yet.
Best we can do at this stage is use the 'agility mod' listed somewhere in the mods section (Sorry no link handy here).
IT alters the jump range parameter somewhat and I found it does allow you to climb certain very specific building types of which
there are few in the game, very few. SO far I've found two building types that allow this to work.
I suspect the few buildings you can climb had improper edits in the core code and somehow were flagged' climbable'. Or, bugs.
I'm no coder, but I"m guessing it would take some hard core assembly language genius to dissemble the core code to find the places
to edit for all building types...now I'm over my pay grade. Full stop.