Player Character Conversion Guide
When I run open table games, I like to encourage folk to bring in characters from ongoing campaigns. My game runs on an OD&D basis with some changes. I call it NatheQuest, but it's really not too complicated or special or anything.
While it's simplest if they are from an old school D&D or similar game, we can also adapt characters from whatever other system (GURPS, Battletech, Burning Wheel (lol), whatever...)
Direct Translations
Characters from roughly compatible systems are easy to translate. They are treated in all ways as a character of the same level and class but instead with NatheQuest rules.
This may result in some differences. For example, a B/X Fighter has d8 for their hit die while I use d6. That's about an average of 1HP per level difference so, while romping around in NatheQuest we can penalize it a bit if their HP is far over the average or possible HP values.
Attack bonus is easy to translate: simply use the NatheQuest values rather than the source game. Equipment and armour is easy to translate directly by simply following my usual procedures (i.e, a one handed weapon deals d6 damage, a heavy weapon deals d6+1 damage, etc).
Characters like Thieves can maintain their Thief Skill odds from their origin game as they likely don't differ that much. Spellcasters can bring whatever spells they like, but their exact effect is up to my preference. Where there is an equivalent spell in my own spell list (i.e, Sleep, Fire Ball, etc) then I will simply default to my own rules and procedures.
Magic items will generally maintain whatever their ordinary effect is.
For classes not in NatheQuest, such as Paladins, Rangers, Psychomancers, or whatever GLOG class, it should be okay to translate it like the above classes while keeping their unique abilities intact. Should anything need to be changed or limited that can be negotiated and figured out when relevant. By adjusting HP, attack bonus and damage to match NatheQuest norms most of the balancing will be done.
Attribute bonuses are limited to +1 in NatheQuest, so that's simple. If your character qualifies for an attribute bonus in their home game then they get the +1 to the relevant score.
Indirect Translations
Things get more tricky if you are trying to bring a GURPS character in, or whatever. For this, simply join the game 10-20 minutes beforehand so we can discuss core elements of the character and move things over.
Lots of stuff is deceptively simple, though. If your character wears Chain, we know that means AC7. If he's got a blessed sword we can assume its roughly equivalent to a +1 Sword, and so on. Nailing the key aspects of a character and triangulating the level are the tricky bits.
For characters from much more powerful systems, like 5e, we can just shrink them down to their best old school equivalent. Your Fighter is just a Fighter. If they can cast some spells, give 'em a few spell slots. The important thing is to work out between the player and referee.
XP Gain
XP gain will be handled as such: you gain XP at the typical NatheQuest rates. If XP systems and requirements are pretty similar then just carry over the XP earned directly. If your home game ref is okay with applying those experience points, go for it.
Some games like 5e have much lower level requirements. In that case, you'll earn a fraction of XP based on the difference between NatheQuest XP requirements and the homegame.
For games like GURPS or FATE with no XP system we'll just use the home system's assumptions for translating XP. No need to overthink it or create exact conversions for every character.
As always, it's important for the ref and player to come to a reasonable compromise regarding the character and their translation. If a character is super powerful, or whatever, they also may not be appropriate for a given adventure.
I think that's about all that needs to be said. The process doesn't need to be particularly onerous, or anything, and I'm hoping to see some more characters from other campaigns. This is something that I think is very fun conceptually, but players have mostly seemed hesitant to do so.