In my own campaign, all PC's are allowed to have a starting Trait that represents life experiences before becoming an 'adventurer'.
An example would be:
UNUSUAL BACKGROUND [TRAIT]
You defy stereotypes.
Prerequisite: Appropriate background.
Benefit: Because of your unusual circumstances, your training in your chosen profession was broader than is usual for your class. Pick three skills which are not class exclusive which you can logically join as part of an invented background. These are considered class skills of your starting class. Some examples to fire the imagination follow:
Barbarian: Listen, Move Silently and Survival
Circus Acrobat: Balance, Perform and Tumble
Courtesan: Bluff, Escape Artist and Perform
Gladiator: Bluff, Disguise, and Tactics
Gutter Mage: Bluff, Hide, and Sleight of Hand
Merchant: Appraise, Diplomacy and Knowledge (Computation and Ledgers)
Martial Artist: Balance, Concentration and Tumble
Nomad: Knowledge (History & Geography), Ride, Survival
Officer: Knowledge (Computation and Ledgers), Leadership, and Tactics
Pirate: Boating, Navigation, and Use Rope
Sapper: Knowledge (Architecture and Engineering), Knowledge (Geology & Mining), Listen
Thug: Hide, Intimidate, and Move Silently
Town Watch: Sense Motive, Spot, and Use Rope
Upper Class: Appraise, Diplomacy, and Knowledge (Customs and Heraldry)
In general the issue you are going to run into is balance. First, typically this additional background is functionally equivalent to starting the game at a higher level. If the character really has been doing a lot with their life before the campaign starts, they probably aren't 1st level and their advantages can be explained by having them be higher level. D&D's mechanics historically set up a 'zero to hero' approach where all starting characters are expected to be newly minted journeymen (at least in some sense, even if just 'father taught me...' or 'I learned in the school of hard knocks...'), having just completed some apprenticeship but not yet acquired any practical experience. It's not suitable for backgrounds of any other sort really.
If your modifications to the starting characters are not very minor, one problem you are going to run into is the problem observed in say Warhammer Fantasy Gaming, where some backgrounds are much more preferable than others. If you start loading down a background template with small benefits, pretty soon you are going to be in a situation where having a background as a gladiator or prince is a lot more useful and exposed you to more educational opportunities than having a background as a ragpicker or a greenseller. This is the reason traits like Unusual Background are nice, because everyone is basically getting the same benefit. Likewise, anything balanced with Unusual Background is probably balanced. It's not a 'sexy' approach, but it is very functional.
But, as is typical in the house rules forum, I can't really help you because I'm not sure what you are expecting to see or what problem you are trying to solve. If you are more specific about what you want, I can probably help.
