Yes it is, and we have done so – but you have to be careful to ensure that some of the core assumptions of the rules are addressed. The campaign system is really a scenario generation system, with the games loosely linked together by their outcomes. These outcomes do three things: they give you money to finance your progress on the career ladder, they tell you when the next raid will be, and they sometimes allow you to add reinforcements (either in partial units, whole units, or quality upgrades).
The scenario generation system is obviously built around the qualities of the two opposing forces. I found that simply importing other rules that didn't mimic their qualities very well resulted in lopsided games.
The starting forces are imbalanced in the sense that the Saxons have a better quality force (2 elites and 3 warriors), while the Britons have a larger but poorer quality force (1 elite, 2 warriors, 3 levy), and both have a pretty useless archer skirmisher unit. The Britons can overcome this quality difference on the defense by using shield wall, which is fairly immobile. However, other than that, both sides essentially move and fight the same.
The rules then use cards for two purposes: first for random movement order of commands, and second for the players to use hands of combat advantage cards – but an added advantage of these cards is that the Briton player can accumulate Pursuit type cards that will effect the level of victory if not countered by Saxon Retreat cards.
So, you need rules that will give about the same level of combat and movement ability (the movement part is just as important), allows for a minor but important shield wall effect on the defense, allows for adding reinforcements in both number and quality, and ideally handles random unit/force activation by leader. Then, you have to decide whether to use the combat cards system and adapt them to your rules, or whether you want to abandon the Pursuit/Retreat mechanic, perhaps in favor of some other indicator of Briton pursuit.
Phew! As an example, we really like Neil Thomas' Ancient and Medieval Warfare rules, and they are so simple that I thought it would be easy to import them. However, I did so without really addressing the issues above, with the result that the Saxons enjoyed much better movement in rough terrain, the Briton shieldwall was overwhelmingly effective, and the previously useless archer unit became a unit killer. It's not that the rules themselves don't make sense, it's just that their assumptions – designed around an open field battle – didn't really work for the Dux Brit scenario generation system.
I hope all that makes sense!?
Cheers,
Christopher
tackles a 
