In the Diplomacy board game, can player A issue a support order for a player B unit? Let's say player B has a hold order for his unit. Can player A use that unit for support in an attack? Basically, at any point under any circumstance can I "commandeer" an opponents unit to support my own attack, and in essence cut that support if he is also using it in an attack?
No, you can only give orders for your own units.
The first sentence of paragraph 2, "Order writing phase" in the rules is, emphasis mine,
Each player secretly writes "orders" for each of his/her own units on a slip of paper.
A nation can support another nation's units. You just need to coordinate and issue the correct orders. Remember, you need to be specific with your support.
Army in X supports army moving from Y into Z.
You can only send orders for your own units. You can use diplomacy to request another player to issue an order, but they are not required to do so.
You can issue an order to support a unit of another player (both move and hold). You can also convoy a unit of another nation.
But you can not support a unit that is attacking you.
The way to obtain the support of Player B's unit is to convince player B to write out the support order on your behalf.
Let's say you are France, allied with England against Germany. On the second turn, you move A Burgundy to Belgium, opposed only by a German fleet, Netherlands to Belgium. You want the support of the English fleet in the North Sea.
Even though you and England are allies, you can't write orders on his behalf. Instead, you must persuade him to write, F North Sea supports A Burgundy to Belgium on his own order card.
This is a good time to find out if England really is your ally. Maybe his order will be, instead, F North Sea supports F Netherlands to Belgium.