It looks like a set phrase. You cannot change it to "хоть убавляй", "хоть отнимай" or "хоть уменьшай".
But, generally speaking, it is not the only "хоть + imperative" construction in Russian. For example, widely used "хоть убей" means "for the life of..." and so are nearly synonymous "хоть лопни" and "хоть тресни". The construction "хоть кол на голове теши" is usually used to denote somebody who does not understand obvious things or ignores accepted rules - "Он не понимает, что его могут ограбить, хоть кол на голове теши" - "He does not accept the idea that he might be robbed, despite multiple attempts to explain to him".
One one might construct other clauses to denote practical impossibility or failed attempts to achieve something, even provided that certain extreme condition would be fulfilled. "Если не изучать грамматику, то не заговоришь на иностранном языке, хоть выучи наизусть десять словарей" - "If one does not study grammar, he/she will not speak a foreign language, even provided he/she had learnt ten dictionaries by heart".
There is another meaning of the construction "хоть <не> + imperative". It is used to denote the deeds that one might be forced to attempt to avoid negative outcome or vice versa might be forced not to attempt because it seems that they will result in highly probable fail or negative outcome.
"На улице холодно, хоть не выходи из дому" - "It is cold outside, I think it would be better not to go outside".
"Мою квартиру ограбили за три раза прошедший месяц, хоть переселяйся в другой район" - "My apartment was burglarized three times last month. I feel like I am forced to move to another neighbourhood".