ad hoc: Created or done for a particular purpose as necessary.
impromptu: Done without being planned or rehearsed
Strictly speaking, Kris' comment is correct. The two expressions have different meanings and are not interchangeable. If every speaker of English would just acknowledge this and use the expressions in their correct sense, the world would be a better place and this question would be off-topic.
However... as with many expression, especially those of foreign origin, the world is not such a beautiful place.
Many people use the two expression interchangeably, and if you are told a meeting room can be used for ad hoc meetings, the speaker usually means any impromptu meeting.
Well, in fact, a lot of impromptu meetings are probably ad hoc:
Guys, the servers crashed again over the weekend, let's get together with the admins and the hardware boys to get to the core of this problem!
If 15 minutes later, there are ten people in a meeting room discussing this problem, they are having both an ad hoc and an impromptu meeting!
On the other hand, you could plan an ad hoc meeting months in advance:
Let's get the CFO's of the twenty biggest companies together next to discuss the impact of new taxation proposals.
That meeting would be ad-hoc, but not impromptu.
Since there is a lot of overlap, it does happen that people get sloppy in the use of the expressions. Of course, I advocate being careful and using the correct expression!
So if you have meeting rooms available that do not need to be booked beforehand, do not reserve them for ad hoc meetings, but for impromptu meetings.
(Personally, I have seen ad hoc used more often in this context!)