WHY?!?
Do you know why REALLY want to use a DH fork?
Are your descents on paths that are THAT MUCH STEEPER than your current fork can accommodate OR do you just want something with more travel?
The only reason that I would use a DH fork on a non-DH frame is because it was the only fork I had at the moment ... which is to say that I wouldn't ...
Consider that the bike's head tube & handlebars will probably be almost 3" higher ...
Unless you are going for the easy-rider effect with your hands in the air when you are not descending, then you may find the compromise of your 'new' riding position may be unacceptable ... especially, when climbing ... OR not?!?
If you are STILL inclined toward your hypothtical option, then you may want to buy a stem riser or two which you can stack on your current steerer to see if you can live with the higher handlebars ... OR, of course, you could remove ANY-and-ALL spacers and/or install a 0º/no-rise handlebar.
BTW. You may need to change your seatpost to one which has ZERO setback so that you can adjust your saddle's fore-aft position as needed, too. If you are already using a seatpost with zero setback then maybe you should reconsider the entire need & subsequent alteration.