We went through this whole subject awhile back.
The short story is... Yes you can use it.
The main issue is there is no standard testing to determine viscosity... Even 10w fork oil from one maker may not be exactly the same as 10w from another. ATF can be anywhere from 5w to 15w really, but the point here is that does not matter to you NOW, go ahead and use it.
The oil you use does not really make the forks "soft"...it's the springs that do that. Fork oil effects the dampening ... the rebound + compression speed.
Not knowing anything about how much you weigh or how you got your bike outfitted, I couldn't make a recommendation as to what would be best for you to use oil weight wise, but ATF will work fine in a pinch if you can't get any regular fork oil.....