If all joint owners agree, can I give away joint assets through a Will?

In an ideal situation - Yes, and you can do so if say all the joint owners give so to the same beneficiary(s).

For example, if the husband and wife are joint owners of a property want the "ultimate" beneficiary to be the son. Both parties can put in accordingly as such. So whoever dies, the other party will inherit it, and subsequently when the other party (which is now the sole owner) dies, it will be given to the son.

However, which any additional agreement, this relies on both parties "keeping their promise". Practically, say if the wife dies earlier, the husband can edit the Will to give to someone else instead. To prevent this, both parties might want consider having a separate legal document drafted.

You can also consider "converting" it to a non-joint type of ownership (e.g. owning a percentage share or sole ownership), but then you would only be able to give away your share of the asset.