I'm trying to get my fiancée interested in playing Warhammer games. She's interested in fantasy and I'm interested in the sci-fi 40k branch of the game, plus I've already got an army in the making. If we got a fantasy army, is there any adapter between the two rulesets?
Unfortunately they are completely different rulesets. While they have a few similar mechanics(rolling to hit/wound or making characteristic tests), the structure of the games do not synch up. Turn structure is different. Ranged weapons, saves, movement, and combat are all different. Applying some fairly common rules in one system to the other can have game breaking results.
Even if you did build a bridge between the two systems the armies would just not play well against each other. Stats aren't balanced between the two systems. They have different baselines(Armor saves in fantasy are mostly 6+, if you have one at all, and in 40k 2+ and 3+ are not uncommon).
The closest you could get would be to have one person play Chaos Demons. The same demon models can be used in fantasy and 40k with the appropriate army book. It's a little awkward because 40k and Fantasy use round and square bases respectively, but most players would give you a pass in friendly games.
Easy. Pick an 40K army from one of the formats and model your stats off of it, but use Warhammer Fantasy models.
For example if your fantasy army has a lot of melee units, why not model them after tyranids like the hormagaunt? Then just use the tyranid rules and costs, 40k rules, and your fantasy models. Fluffing it up is up to you! "Oh, no he didn't mutate to get his stat bonus, he reached into his bag of tricks!"
I did manage to mix the two, but not in the way you're probably thinking.
I had a WFRP campaign that I sent forward to the 40K time period.
It was quite straightforward to adapt a 40K unit into a WFRP NPC.
Probably not the kind of a solution you were looking for though.
Still, notable because it worked really well. Very memorable game for all concerned.