If an item was infused by an Artificer, say using the returning weapon infusion, would it be possible to then add some other magic to the weapon. You cannot infuse a magical item, but does that limit still apply after the infusion has been made? An example might be the Hexblade Warlock's Hex Warrior feature. Would this be allowed as the RAW seem to imply it would be.
Adding additional magic to an infused item does not negate the infusion
The requirement for infusing an item is:
Whenever you finish a long rest, you can touch a non-magical object and imbue it with one of your artificer infusions, turning it into a magic item.
Furthermore, several conditions are given for ending an infusion:
Your infusion remains in an item indefinitely, but when you die, the infusion vanishes after a number of days have passed equal to your Intelligence modifier (minimum of 1 day). The infusion also vanishes if you give up your knowledge of the infusion for another one.
[...] If you try to exceed your maximum number of infusions, the oldest infusion immediately ends, and then the new infusion applies.
There is nothing about an infusion ending if some other magic is applied to the infused item. The only requirement is that the item must be non-magical at the time you apply the infusion in order to infuse it in the first place. Beyond that, no further changes to the item will cause the infusion to end. In fact, I don't see any way in the feature's text to voluntarily end an infusion other than infusing enough additional items to exceed your maximum.
No, you can't.
In DnD 5e, there are no rules for enchanting an already-enchanted item. When you create a magic item, you create a specific item; there are no rules for turning e.g. a +1 Longsword into a +2 Longsword, and you can't create something like a +1 Mithral Plate.