Increasing bandit relations

I've been wanting to play as a bandit character, who seeks to profit by attacking the trade caravans of Christians. I don't wish to have hostile relations with any of the bandit/outlaw parties that roam the map, including the viking bandits. Because of this, I've avoided getting into combat with any of these factions.

I haven't been able to boost relations with these factions. I can't seem to join battles to help them out. It's expensive having to bribe them; this DLC is unique in that, when a bandit party attacks me, I lose relations with them if battle starts, it doesn't matter whether they're the aggressor or not.

Is it even possible to have non-hostile relations with the bandit parties?

check modding section for the Tweaks Thread or VC Tweaks Tool. You can enable the option to "join any side in a battle" to help bandits.

you should also consider changing the basic relations with bandits on a new game (as they are negative) to make your life easier.